## 1. Kant's "Answer to Hume"



In the Preface to the *Prolegomena* Kant considers the supposed
science of metaphysics. He states that "no event has occurred
that could have been more decisive for the fate of this science than
the attack made upon it by David Hume" and goes on to say that
"Hume proceeded primarily from a single but important concept of
metaphysics, namely, that of *the connection of cause and
effect*" (4, 257; 7; see the Bibliography for our method of
citation). Over the next few pages Kant defends the importance of
Hume's "attack" on metaphysics against common-sense
opponents such as Thomas Reid, James Oswald, James Beattie, and Joseph
Priestley (all of whom, according to Kant, missed the point of
Hume's problem), and Kant then famously writes (4, 260; 10):




> 
> 
> I freely admit that it was the remembrance of David Hume which, many
> years ago, first interrupted my dogmatic slumber and gave my
> investigations in the field of speculative philosophy a completely
> different direction. 
> 
> 
> 



Thus, it was Hume's "attack" on metaphysics (and, in
particular, on the concept of cause and effect) which first provoked
Kant himself to undertake a fundamental reconsideration of this
(supposed) science.



Later, in SSSS 27-30 of the *Prolegomena*, Kant
returns to Hume's problem and presents his own solution. Kant
begins, in SS 27, by stating that "here is now the place to
remove the Humean doubt from the ground up" (4, 310; 63); and he
continues, in SS 29, by proposing 



> 
> 
> to make a *trial with Hume's* problematic concept (his
> *crux metaphysicorum*), *namely* the concept of cause.
> (4, 312; 65) 
> 
> 
> 



Kant concludes, in SS 30, by stating that we are now in possession
of "a complete solution of the Humean problem" (4, 313;
66)--which, Kant adds, 



> 
> 
> rescues the a priori origin of the pure concepts of the understanding
> and the validity of the general laws of nature as laws of the
> understanding, in such a way that their use is limited only to
> experience, because their possibility has its ground merely in the
> relation of the understanding to experience, however, not in such a
> way that they are derived from experience, but that experience is
> derived from them, a completely reversed kind of connection which
> never occurred to Hume. (ibid.) 
> 
> 
> 



Thus, Kant's "complete solution of the Humean
problem" directly involves him with his whole revolutionary
theory of the constitution of experience by the a priori concepts and
principles of the understanding--and with his revolutionary
conception of synthetic a priori judgments.



Indeed, when Kant first introduces Hume's problem in the Preface
to the *Prolegomena* he already indicates that the problem is
actually much more general, extending to all of the categories of the
understanding (4, 260; 10): 



> 
> 
> I thus first tried whether Hume's objection might not be
> represented generally, and I soon found that the concept of the
> connection of cause and effect is far from being the only one by which
> the understanding thinks connections of things a priori; rather,
> metaphysics consists wholly and completely of them. I sought to secure
> their number, and since this succeeded as desired, namely, from a
> single principle, I then proceeded to the deduction of these concepts,
> on the basis of which I was now assured that they are not derived from
> experience, as Hume had feared, but have sprung from the pure
> understanding. 
> 
> 
> 



Moreover, Kant soon explains, in SS 5, how this more general
problem (common to all the categories and principles of the
understanding) is to be formulated: "How is cognition from pure
reason possible?" (4, 275; 27), or, more specifically,
"How are synthetic a priori propositions possible?" (4,
276; 28).



In the Introduction to the second (B) edition of the *Critique of
Pure Reason* (1787), Kant follows the *Prolegomena* in
formulating what he here calls "the general problem of pure
reason" (B19): "How are synthetic a priori judgments
possible?" And, as in the *Prolegomena*, Kant insists
that the possibility of metaphysics as a science entirely depends on
this problem (ibid.): 



> 
> 
> That metaphysics until now has remained in such a wavering state of
> uncertainty and contradictions is to be ascribed solely to the fact
> that this problem, and perhaps even the distinction between
> *analytic* and *synthetic* judgments, was not thought of
> earlier. Metaphysics stands or falls with the solution of this
> problem, or on a satisfactory proof that the possibility it requires
> to be explained does not in fact obtain. 
> 
> 
> 



Kant then immediately refers to "David Hume, who, among all
philosophers, came closest to this problem"; and he suggests,
once again, that Hume failed to perceive the solution because he did
not conceive the problem in its 



> 
> 
> [full] generality, but rather stopped with the synthetic proposition
> of the connection of the effect with the cause (*principium
> causalitatis*). (ibid.)
> 
> 
> 



It is only in the second edition of the *Critique* that Kant
gives such a prominent place to Hume and his "objection"
to causality, serving to introduce what Kant now calls "the
general problem of pure reason". By contrast, the name of Hume
does not appear in either the Introduction or the Transcendental
Analytic in the first (A) edition (1781): it appears only in the
Transcendental Doctrine of Method at the very end of the book, in a
discussion of "skepticism" versus "dogmatism"
in metaphysics (where Hume's skepticism about causation, in
particular, is finally explicitly discussed). This is not to say, of
course, that implicit references to Hume are not found earlier in the
text of the first edition. Thus, for example, in a preliminary section
to the Transcendental Deduction Kant illustrates the need for such a
deduction with the concept of cause, and in both editions remarks
(A91/B124): 



> 
> 
> Appearances certainly provide cases from which a rule is possible in
> accordance with which something usually happens, but never that the
> succession is *necessary*; therefore, a dignity pertains to the
> synthesis of cause and effect that cannot be empirically expressed at
> all, namely, that the effect does not merely follow upon the cause but
> is posited *through* it and follows *from* it. 
> 
> 
> 



But it is only in the second edition that Kant then goes on to mention
"David Hume" explicitly, as one who attempted to derive
the pure concepts of the understanding from experience (B127): 



> 
> 
> namely, from a subjective necessity arising from frequent association
> in experience--i.e., from *custom*--which is
> subsequently falsely taken for objective. 
> 
> 
> 



This striking difference between the two editions clearly reflects the
importance of the intervening appearance of the
*Prolegomena*.



Given the crucial importance of the *Prolegomena* in this
respect, it is natural to return to Kant's famous remarks in the
Preface to that work, where, as we have seen, Kant says that 



> 
> 
> it was the remembrance of David Hume which, many years ago, first
> interrupted my dogmatic slumber and gave my investigations in the
> field of speculative philosophy a completely different direction. 
> 
> 
> 



It is natural to wonder, in particular, about the precise years to
which Kant is referring and the specific events in his intellectual
development he has in mind. Here, however, we now enter controversial
terrain, where there are basically two competing
alternatives--both of which reflect the circumstance that Kant
could read Hume only in German translation.



Kant might be referring, on the one hand, to the late 1750s to mid
1760s. A translation of Hume's *Enquiry Concerning Human
Understanding* (originally published in 1748) appeared in 1755 and
was widely read in Germany. Kant had almost certainly read this
translation by the mid 1760s, by which time he himself expressed
doubts about whether causal connections could be known by reason alone
and even suggested that they were knowable only by experience. Or, on
the other hand, Kant might be referring to the mid 1770s. After the
*Inaugural Dissertation* appeared in 1770, Kant published
nothing more until the first edition of the *Critique* in 1781.
Meanwhile, a German translation of Beattie's *Essay on the
Nature and Immutability of Truth* (originally published in 1770)
appeared in 1772, where, in particular, Beattie quoted extensively
from Book 1 of Hume's *Treatise of Human Nature*
(originally published in 1739). Thus, in the famous "dogmatic
slumber" passage, Kant might be referring either to the mid
1760s, when he then had a "remembrance" of reading the
translation of Hume's *Enquiry*, or to the mid 1770s,
when he then had a "remembrance" of reading translations
from the
 *Treatise*.[1]



We prefer the first alternative. From this point of view, the decisive
event to which Kant is referring is his reading of Hume's
*Enquiry* (in translation) during the late 1750s to mid 1760s,
and this event, we believe, is clearly reflected in two important
writings of the mid 1760s: the *Attempt to Introduce the Concept of
Negative Magnitudes into Philosophy* (1763) and *Dreams of a
Spirit-Seer Explained by Dreams of Metaphysics* (1766).



In the first (1763) essay Kant introduces the distinction between
"logical grounds" and "real grounds", both of
which indicate a relationship between a "ground" (cause or
reason) and a "consequent" (following from this ground).
Kant explains his problem as follows (2, 202; 239):



> 
> 
> I understand very well how a consequent may be posited through a
> ground in accordance with the rule of identity, because it is found to
> be contained in [the ground] by the analysis of concepts. ...
> [A]nd I can clearly comprehend this connection of the ground with the
> consequent, because the consequent is actually identical with part of
> the concept of the ground .... However, how something may flow
> from another, but not in accordance with the rule of identity, is
> something that I would very much like to have made clear to me. I call
> the first kind of ground a logical ground, because its relation to the
> consequent can be logically comprehended in accordance with the rule
> of identity, but I call the second kind of ground a real ground,
> because this relation indeed belongs to my true concepts, but the
> manner of this [relation] can in no way be estimated. With respect to
> such a real ground and its relation to the consequent, I pose my
> question in this simple form: how can I understand the circumstance
> *that, because something is, something else is to be?* A
> logical consequent is only posited because it is identical with the
> ground.
> 
> 
> 



The fundamental problem with the relationship between a real ground
and its consequent, therefore, is that the consequent is *not*
identical with either the ground or a part of this concept--i.e.,
it is *not* "contained in [the ground] by the analysis of
concepts".



Thus, using his well-known later terminology (from the
*Critique* and the *Prolegomena*), Kant is here saying
that, in the case of a real ground, the relationship between the
concept of the consequent (e.g., an effect) and the concept of the
ground (e.g., a cause) is not one of containment, and the judgment
that the former follows from the latter is therefore not
*analytic*. Moreover, although Kant does not explicitly refer
to Hume in the essay on *Negative Magnitudes*, he proceeds to
illustrate his problem with an example (among others) of the causal
connection in the communication of motion by impact (2, 202; 240):




> 
> 
> A body *A* is in motion, another *B* is at rest in the
> straight line [of this motion]. The motion of *A* is something,
> that of *B* is something else, and, nevertheless, the latter is
> posited through the former. 
> 
> 
> 



Hume famously uses this example (among others) in the *Enquiry*
to illustrate his thesis that cause and effect are entirely distinct
events, where the idea of the latter is in no way contained in the
idea of the former (EHU 4.9; SBN 29): 



> 
> 
> The mind can never possibly find the effect in the supposed cause, by
> the most accurate scrutiny and examination. For the effect is totally
> different from the cause, and consequently can never be discovered in
> it. Motion in the second billiard-ball is a quite distinct event from
> motion in the first; nor is there anything in the one to suggest the
> smallest hint of the other. 
> 
> 
> 



A few lines later Hume describes this example as follows (EHU 4.10;
SBN 29): 



> 
> 
> When I see, for instance, a billiard-ball moving in a straight line
> towards another; even suppose motion in the second ball should by
> accident be suggested to me, as the result of their contact or
> impulse; may I not conceive, that a hundred different events might as
> well follow from the cause? ... All these suppositions are
> consistent and conceivable.
> 
> 
> 



In Kant's second essay from this period, *Dreams of a
Spirit-Seer* (1766), he goes further: he suggests a Humean
solution to the problem he had posed, but did not solve, in the essay
on *Negative Magnitudes*. Kant suggests, more specifically,
that the relation between a real ground and its consequent can only be
given by experience (2, 370; 356):



> 
> 
> It is impossible ever to comprehend through reason how something could
> be a cause or have a force, rather these relations must be taken
> solely from experience. For the rule of our reason extends only to
> comparison in accordance with *identity* and
> *contradiction*. But, in so far as something is a cause, then,
> through *something*, something *else* is posited, and
> there is thus no connection in virtue of agreement to be
> found--just as no contradiction will ever arise if I wish to view
> the former not as a cause, because there is no contradiction [in the
> supposition that] if something is posited, something else is
> cancelled. Therefore, if they are not derived from experience, the
> fundamental concepts of things as causes, of forces and activities,
> are completely arbitrary and can neither be proved nor refuted.
> 
> 
> 



This passage seems clearly to recall the main ideas in section 4, part
1 of Hume's *Enquiry*. After distinguishing between
"relations of ideas" and "matters of fact",
and asserting that the former "are discoverable by the mere
operation of thought" (EHU 4.1; SBN 25), Hume continues (EHU
4.2; SBN 25): 



> 
> 
> Matters of fact, which are the second objects of human reason, are not
> ascertained in the same manner; nor is our evidence of their truth,
> however great, of a like nature with the foregoing. The contrary of
> every matter of fact is still possible; because it can never imply a
> contradiction .... 
> 
> 
> 



Hume then explains that: "all reasonings concerning matters of
fact seem to be founded on the relation of *Cause and
Effect*" (EHU 4.4; SBN 26) and adds (EHU 4.6; SBN 27): 



> 
> 
> I shall venture to affirm, as a general proposition, which admits of
> no exception, that the knowledge of this relation is not, in any
> instance, attained by reasonings *a priori*; but arises
> entirely from experience, when we find that any particular objects are
> constantly conjoined with each other. 
> 
> 
> 



Finally (EHU 4.10; SBN 29): 



> 
> 
> And as the first imagination or invention of a particular effect, in
> all natural operations, is arbitrary, where we consult not experience;
> so must we also esteem the supposed tye or connexion between the cause
> and effect, which binds them together, and renders it impossible that
> any other effect could result from the operation of that cause. 
> 
> 
> 



Thus, although Kant does not explicitly mention Hume in *Dreams of
a Spirit-Seer*, the parallels with Hume's *Enquiry*
are striking
 indeed.[2]



Kant does not endorse a Humean solution to the problem of the relation
between cause and effect in the critical period (beginning with the
first edition of the *Critique* in 1781): he does not (as he
had in *Dreams of a Spirit-Seer*) claim that this relation is
derived from experience. Instead (as we have seen) Kant takes
Hume's problem of causality to be centrally implicated in the
radically new problem of synthetic a priori judgments. Yet the latter
problem, in turn, clearly has its origin in Kant's earlier
discussion (in the essay on *Negative Magnitudes* and
*Dreams of a Spirit-Seer*) of the apparently mysterious
connection between a real ground (or cause) and its consequent (or
effect). Just as Kant had earlier emphasized (in these pre-critical
works) that the consequent of a real ground is not contained in it,
and thus does not result by "the analysis of concepts",
Kant now (in the critical period) maintains that the concept of the
effect cannot be contained in the concept of the cause and,
accordingly, that a judgment relating the two cannot be analytic. Such
a judgment, in Kant's critical terminology, must now be
synthetic--it is a judgment in which "the connection of the
predicate with the subject ... is thought without
identity", where 



> 
> 
> a predicate is added to the concept of the subject which is by no
> means thought in it, and which could not have been extracted from it
> by any analysis. (A7/B10-11) 
> 
> 
> 



The crucial point about a synthetic *a priori* judgment,
however, is that, although it is certainly not (as a priori) derived
from experience, it nonetheless extends our knowledge beyond merely
analytic judgments.



It therefore becomes clear why, in the Introduction to the second
edition of the *Critique*, Kant says of the crucial problem of
synthetic a priori judgments that 



> 
> 
> this problem, and perhaps even the distinction between
> *analytic* and *synthetic* judgments, was not thought of
> earlier, 
> 
> 
> 



and then explicitly names "David Hume, who, among all
philosophers, came closest to this problem" (B19). It also
becomes clear why, in the Preface to the *Prolegomena*, Kant
explains Hume's problem as follows (4, 257; 7):



> 
> 
> Hume proceeded primarily from a single but important concept of
> metaphysics, namely, that of *the connection of cause and
> effect* ... , and he challenged reason, which here pretends
> to have generated this concept in her womb, to give him an account of
> by what right she thinks that something could be so constituted that,
> if it is posited, something else must necessarily also be posited
> thereby; for this is what the concept of cause says. He proved
> indisputably that it is completely impossible for reason to think such
> a connection a priori and from concepts [alone] (for this [connection]
> contains necessity); but it can in no way be comprehended how, because
> something is, something else must necessarily also be, and how,
> therefore, the concept of such a connection could be introduced a
> priori.
> 
> 
> 



Thus here, in the *Prolegomena*, Kant describes what he calls
Hume's "challenge" to reason concerning
"*the connection of cause and effect*" in precisely
the same terms that he had himself earlier used, in the 1763 essay on
*Negative Magnitudes* and the 1766 *Dreams of a
Spirit-Seer*, to pose a fundamental problem about the relation of
a real ground (as opposed to a logical ground) to its consequent.



What is most important, however, is the official solution to
Hume's problem that Kant presents in SS 29 of the
*Prolegomena*. This solution depends on the distinction between
"judgments of perception" and "judgments of
experience" which Kant has extensively discussed in the
preceding sections. In SS 18 Kant introduces the distinction as
follows (4, 298; 51):



> 
> 
> *Empirical judgments, in so far as they have objective
> validity,* are **judgments of experience**; they,
> however, *in so far as they are only subjectively valid*, I
> call mere **judgments of perception**. ... All of
> our judgments are at first mere judgments of perception: they are
> valid merely for us, i.e., for our subject, and only afterwards do we
> give them a new relation, namely to an object, and we intend that [the
> judgment] is supposed to be also valid for us at all times and
> precisely so for everyone else; for, if a judgment agrees with an
> object, then all judgments about the same object must also agree among
> one another, and thus the objective validity of the judgment of
> experience signifies nothing else but its necessary universal
> validity.
> 
> 
> 



Then, in SS 22, Kant emphasizes that the pure concepts of the
understanding or categories function precisely to convert mere
(subjective) perceptions into objective experience by effecting a
"*necessary* unification" of them (4, 305; 58):




> 
> 
> Therefore, the pure concepts of the understanding are those concepts
> under which all perceptions must first be subsumed before they can
> serve as judgments of experience, in which the synthetic unity of
> perceptions is represented as necessary and universally
>  valid.[3]
> 
> 
> 
> 



Here is how Kant formulates his solution in SS 29 (4, 312;
65):



> 
> 
> In order to make a *trial with Hume's* problematic
> concept (his *crux metaphysicorum*), *namely* the
> concept of cause, first, there is given to me a priori, by means of
> logic, the form of a conditional judgment in general, namely, to use a
> given cognition as ground and the other as consequent. It is possible,
> however, that a rule of relation is found in perception which says
> that a given appearance is constantly followed by another (but not
> conversely); and this is a case for me to employ the hypothetical
> judgment and, e.g., to say: if a body is illuminated sufficiently long
> by the sun, then it becomes warm. Here, there is certainly no
> necessity of connection as yet, and thus [not] the concept of cause.
> However, I continue and say that, if the above proposition, which is
> merely a subjective connection of perceptions, is to be a judgment of
> experience, then it must be viewed as necessary and universally valid.
> But such a proposition would be: the sun is through its light the
> cause of heat. The above empirical rule is now viewed as a
> law--and, in fact, not as valid merely of appearances, but
> [valid] of them on behalf of a possible experience, which requires
> completely and thus necessarily valid rules.
> 
> 
> 



All the elements from Kant's earlier discussion of causality in
the essays on *Negative Magnitudes* and *Dreams of a
Spirit-Seer* seem to be present here. Kant begins with the purely
logical relation between ground and consequent. Since, in the case of
the concept of cause, we are dealing with what Kant had earlier called
a *real* ground, Kant holds that we need a synthetic rather
than merely analytic connection between the two. The most obvious
thought, which Hume had defended in the *Enquiry* (and,
apparently following Hume, Kant himself had defended in *Dreams of
a Spirit-Seer*) is that "experience" (in the Humean
sense) is the basis for this connection in so far as one perception is
found to be "constantly conjoined" with another. Now,
however, in the critical period, Kant introduces a revolutionary new
concept of "experience" which is explicitly opposed to
mere constant conjunctions among perceptions in being "necessary
and universally valid"--in particular, "experience is
possible only by means of the representation of a necessary connection
of perceptions" (B218).



In Kant's example from SS 29 of the *Prolegomena*,
then, we begin from a mere subjective "empirical rule":
that the perception of an illuminated stone is constantly followed by
the perception of heat; and we then convert this "empirical
rule" into an objective law according to which the very same
relationship is now viewed as "necessary and universally
valid". This transformation is effected by the addition of the a
priori concept of causality: "the sun is through its light the
cause of heat". It is in precisely this way, more generally,
that the categories or pure concepts of the understanding relate to
experience: 



> 
> 
> not in such a way that they are derived from experience, but that
> experience is derived from them, a completely reversed kind of
> connection which never occurred to Hume. (SS 30: 4, 313; 66)
> 
> 
> 



We shall devote the rest of this article to clarifying Kant's
solution and its relationship with Hume's conception of
causation. For now, we simply note an important difficulty Kant
himself raises in the *Prolegomena*. Whereas the concept of
causality is, for Kant, clearly a priori, he does not think that
particular causal laws relating specific causes with specific effects
are all synthetic a priori--and, if they are not a priori, how
can they be necessary? Indeed, Kant illustrates this difficulty, in a
footnote to SS 22, with his own example of the sun warming a stone
(4, 305; 58):



> 
> 
> But how does this proposition, that judgments of experience are
> supposed to contain necessity in the synthesis of perceptions, agree
> with my proposition, urged many times above, that experience, as a
> posteriori cognition, can yield only contingent judgments? If I say
> that experience teaches me something, I always mean only the
> perception that lies within in it, e.g., that heat always follows the
> illumination of the stone by the sun. That this heating results
> necessarily from the illumination by the sun is in fact contained in
> the judgment of experience (in virtue of the concept of cause); but I
> do not learn this from experience, rather, conversely, experience is
> first generated through this addition of the concept of the
> understanding (of cause) to the perception.
> 
> 
> 



In other words, experience in the Humean sense teaches me that heat
always (i.e., constantly) follows the illumination of the stone by the
sun; experience in the Kantian sense then adds that: 



> 
> 
> the succession is *necessary*; ... the effect does not
> merely follow upon the cause but is posited *through* it and
> follows *from* it. (A91/B124) 
> 
> 
> 



But what exactly does this mean?


## 2. Induction, Necessary Connection, and Laws of Nature



Kant formulates a crucial distinction between "strict" and
"comparative" universality in SS II of the
Introduction to the second edition of the *Critique*
(B3-4):



> 
> 
> Experience never gives its judgments true or strict, but merely
> assumed or comparative *universality* (through induction), so
> that, properly speaking, it must be formulated: so far as we have
> observed until now, no exception has been found to this or that rule.
> If, therefore, a judgment is thought with strict universality, i.e.,
> so that no exception at all is allowed to be possible, then it is not
> derived from experience, but is valid absolutely a priori. Empirical
> universality is thus only an arbitrary augmentation of validity from
> that which is valid in most cases to that which is valid in
> all--as, e.g., in the proposition: all bodies are heavy. By
> contrast, where strict universality essentially belongs to a judgment,
> this [universality] indicates a special source of cognition for [the
> judgment], namely a faculty of a priori cognition. Necessity and
> strict universality are thus secure criteria of an a priori cognition,
> and also inseparably belong together.
> 
> 
> 



Kant then explicitly links this distinction to Hume's discussion
of causality in the following paragraph (B5): 



> 
> 
> The very concept of cause so obviously contains the concept of a
> necessity of the connection with an effect and a strict universality
> of the rule, that the concept [of cause] would be entirely lost if one
> pretended to derive it, as Hume did, from a frequent association of
> that which happens with that which precedes, and [from] a thereby
> arising custom (thus a merely subjective necessity) of connecting
>  representations.[4]
> 
> 
> 
> 



Moreover, in the second edition (as we have seen) Kant also goes on to
name Hume explicitly, as one who attempted to derive the concept of
causality 



> 
> 
> from a subjective necessity arising from frequent association in
> experience--i.e., from *custom*--which is
> subsequently falsely taken for objective. (B127) 
> 
> 
> 



It appears, therefore, that Kant's discussion, in SS 29 of
the *Prolegomena*, of how, by the addition of the concept of
cause, we convert a mere subjective "empirical rule" into
an objective law (which is "necessary and universally
valid"), is not only indebted to Hume for the insight that the
connection between cause and effect is synthetic rather than analytic,
it is also indebted to Hume's discussions of the problem of
induction (in section 4, part 2 of the *Enquiry*) and of the
idea of necessary connection (in section 7). Kant agrees with Hume
that the idea of necessary connection is in fact an essential
ingredient in our idea of the relation between cause and effect; Kant
agrees, in addition, that, if all we had to go on were a purely
inductive inference from observed constant conjunctions, the inference
from comparative to strict universality would not be legitimate, and
the presumed necessary connection arising in this way (i.e., from
custom) would be merely subjective.



Section 4 of the *Enquiry* is entitled "Sceptical Doubts
Concerning the Operations of the Understanding". In part 1 of
this section (as we have already seen) Hume maintains that the idea of
the effect is never contained in the idea of the cause (in
Kant's terminology, the relation is not analytic), and thus,
according to Hume, it is never knowable a priori. We therefore need
experience in the Humean sense in order to make any causal
claims--that is, the observation of an event of one type
*A* constantly followed by an event of another type *B*.
Otherwise (as we have also seen) any event could follow any other (EHU
4.10; SBN 29): 



> 
> 
> And as the first imagination or invention of a particular effect, in
> all natural operations, is arbitrary, where we consult not experience;
> so must we also esteem the supposed tye or connexion between the cause
> and effect, which binds them together, and renders it impossible that
> any other effect could result from the operation of that cause. 
> 
> 
> 



Note that Hume is here supposing that, in our idea of the relation
between cause and effect, the "tye or connexion ... which
binds them together" is necessary ("it is impossible that
any other effect could result"). In the corresponding section of
the *Treatise*, Book 1, part 3, section 2 ("Of
probability; and of the idea of cause and effect"), Hume makes
this completely explicit (T 1.3.2.11; SBN 77): 



> 
> 
> Shall we then rest contented with these two relations of contiguity
> and succession, as affording a compleat idea of causation? By no
> means. An object may be continuous and prior to another, without being
> consider'd as its cause. There is a NECESSARY CONNEXION to be
> taken into consideration; and that relation is of much greater
> importance, than any of the other two above-mention'd.
> 
> 
> 



In the *Enquiry*, section 4, part 2, Hume presents his famous
skeptical argument concerning causation and induction. Since we need
"experience" (i.e., the observation of constant
conjunctions) to make any causal claims, Hume now asks (EHU 4.14; SBN
32): "*What is the foundation of all conclusions from
experience?*" The conclusion from an experience of constant
conjunction is an inference to what has not yet been observed from
what has already been observed, and Hume finds an unbridgeable gap
between the premise (summarizing what we have observed so far) and the
(not yet observed) conclusion of this inference (EHU 4.16; SBN 34):




> 
> 
> These two propositions are far from being the same, *I have found
> that such an object has always been attended with such an effect*,
> and *I foresee, that other objects, which are, in appearance,
> similar, will be attended with similar effects*. 
> 
> 
> 



Hume concludes that this inference has no foundation in the
understanding--that is, no foundation in what he calls
 "reasoning".[5]
 How does Hume arrive at this position?



All our inductive inferences--our "conclusions from
experience"--are founded on the supposition that the course
of nature is sufficiently uniform so that the future will be
conformable to the past (EHU 4.21; SBN 37-38): 



> 
> 
> For all inferences from experience suppose, as their foundation, that
> the future will resemble the past .... If there be any suspicion,
> that the course of nature may change, and that the past may be no rule
> for the future, all experience becomes useless, and can give rise to
> no inference or conclusion. 
> 
> 
> 



Therefore, what Hume is now seeking, in turn, is the foundation in our
reasoning for the supposition that nature is sufficiently uniform.



Section 4, part 1 of the *Enquiry* distinguishes (as we have
seen) between reasoning concerning relations of ideas and reasoning
concerning matters of fact and existence. Demonstrative reasoning
(concerning relations of ideas) cannot establish the supposition in
question, 



> 
> 
> since it implies no contradiction, that the course of nature may
> change, and that an object, seemingly like those which we have
> experienced, may be attended with different or contrary effects. (EHU
> 4.18; SBN 35) 
> 
> 
> 



Moreover, reasoning concerning matters of fact and existence cannot
establish it either, since such reasoning is always founded on the
relation of cause and effect, the very relation we are now attempting
to found in reasoning (EHU 4.19; SBN 35-36): 



> 
> 
> We have said, that all arguments concerning existence are founded on
> the relation of cause and effect; that our knowledge of that relation
> is derived entirely from experience; and that all our experimental
> conclusions proceed upon the supposition, that the future will be
> conformable to the past. To endeavour, therefore, the proof of this
> last proposition by probable arguments, or arguments regarding
> existence, must be evidently going in a circle, and taking that for
> granted, which is the very point in
>  question.[6]
> 
> 
> 



Although Hume has now shown that there is no foundation for the
supposition that nature is sufficiently uniform in reasoning or the
understanding, he goes on, in the following section 5 of the
*Enquiry* ("Skeptical Solution of these Doubts"),
to insist that we are nonetheless always determined to proceed in
accordance with this supposition. There is a natural basis or
"principle" for all our arguments from experience, even if
there is no ultimate foundation in reasoning (EHU 5.4-5; SBN
42-43):



> 
> 
> And though [one] should be convinced, that his understanding has no
> part in the operation, he would nonetheless continue in the same
> course of thinking. There is some other principle, which determines
> him to form such a conclusion. This principle is CUSTOM or HABIT. For
> wherever the repetition of any particular act or operation produces a
> propensity to renew the same act or operation, without being impelled
> by any reasoning or process of the understanding; we always say, that
> this propensity is the effect of *Custom*. By employing that
> word, we pretend not to have given the ultimate reason of such a
> propensity. We only point out a principle of human nature, which is
> universally acknowledged, and which is well known by its
>  effects.[7]
> 
> 
> 



In section 7 of the *Enquiry* ("On the Idea of Necessary
Connexion"), after rejecting the received views of causal
necessity, Hume explains that precisely this custom or habit also
produces our idea of necessary connection (EHU 7.28; SBN 75):



> 
> 
> It appears, then, that this idea of a necessary connexion among events
> arises from a number of similar instances which occur of the constant
> conjunction of these events; nor can that idea ever be suggested by
> any one of these instances, surveyed in all possible lights and
> positions. But there is nothing in a number of instances, different
> from every single instance, which is supposed to be exactly similar;
> except only, that after a repetition of similar instances, the mind is
> carried by habit, upon the appearance of one event, to expect its
> usual attendant, and to believe that it will exist. This connexion,
> therefore, which we *feel* in the mind, this customary
> transition of the imagination from one object to its usual attendant,
> is the sentiment or impression, from which we form the idea of power
> or necessary connexion.
> 
> 
> 



Thus, the custom or habit to make the inductive inference not only
gives rise to a new idea of not yet observed instances resembling the
instances we have already observed, it also produces a feeling of
determination to make the very inductive inference in question. This
feeling of determination, in turn, gives rise to a further new idea,
the idea of necessary connexion, which has no resemblance whatsoever
with anything we have observed. It is derived from an
"impression of reflection" (an internal feeling or
sentiment), not from an "impression of sensation" (an
observed instance before the mind), and it is in precisely this sense,
for Hume, that the idea of necessary connection is merely subjective.
Hume emphasizes that this is a "discovery" both "new
and extraordinary", and that it is skeptical in character (EHU
7.28-29; SBN 76): 



> 
> 
> No conclusions can be more agreeable to scepticism than such as make
> discoveries concerning the weakness and narrow limits of human reason
> and capacity. And what stronger instance can be produced of the
> surprising ignorance and weakness of the understanding, than the
> present? For surely, if there be any relation among objects, which it
> imports to us to know perfectly, it is that of cause and effect.
> 
> 
> 



Kant agrees with Hume that neither the relation of cause and effect
nor the idea of necessary connection is given in our sensory
perceptions; both, in an important sense, are contributed by our mind.
For Kant, however, the concepts of both causality and necessity arise
from precisely the operations of our understanding--and, indeed,
they arise entirely a priori as pure concepts or categories of the
understanding. It is in precisely this way that Kant thinks that he
has an answer to Hume's skeptical problem of induction: the
problem, in Kant's terms, of grounding the transition from
merely "comparative" to "strict universality"
(A91-92/B123-124). Thus in SS 29 of the
*Prolegomena*, as we have seen, Kant begins from a merely
subjective "empirical rule" of constant conjunction or
association among our perceptions (of heat following illumination by
the sun), which is then transformed into a "necessary and
universally valid law" by adding the a priori concept of
cause.



At the end of our discussion in section 1 above we saw that there is a
serious difficulty in understanding what Kant intends here--a
difficulty to which he himself explicitly calls attention. Kant does
not think that the particular causal law that "the sun is
through its light the cause of heat" is itself a synthetic a
priori truth. Indeed, the very same difficulty is present in our
discussion at the beginning of this section. For, what Kant is saying
in SS II of the second edition of the Introduction to the
*Critique* is that necessity and strict universality are
"secure criteria of an *a priori* cognition" (B4;
emphasis added). More specifically (B3): 



> 
> 
> Experience in fact teaches us that something is constituted thus and
> so, but not that it cannot be otherwise. Hence, if ... a
> proposition is thought together with its *necessity*, then it
> is an a priori judgment. 
> 
> 
> 



Yet, once again, Kant does not think that particular causal laws
relating specific causes to specific effects are all (synthetic) a
priori. Accordingly, when Kant provides examples of (synthetic) a
priori cognitions in the immediately following paragraph, he cites the
synthetic a priori principle of the Second Analogy of Experience
("All alterations take place in accordance with the law of the
connection of cause and effect" [B232]) rather than any
particular causal law (B4-5): 



> 
> 
> Now it is easy to show that there actually are such judgments in human
> cognition which are necessary and in the strictest sense universal,
> and therefore purely a priori. If one wants an example from the
> sciences, then one need only take a look at any of the propositions of
> mathematics. If one wants such an example from the most common use of
> the understanding, then the proposition that every alteration must
> have a cause can serve.
> 
> 
> 



On the basis of this important passage, among others, the majority of
twentieth-century English-language commentators have rejected the idea
that Kant has a genuine disagreement with Hume over the status of
particular causal laws. One must sharply distinguish between the
general principle of causality of the Second Analogy--the
principle that every event *b* must have a cause
*a*--and particular causal laws: particular instantiations
of the claim that all events of type *A* must always be
followed by events of type *B*. The former is in fact a
synthetic a priori necessary truth holding as a transcendental
principle of nature in general, and this principle is explicitly
established in the Second Analogy. But the Second Analogy does not
establish, on this view, that particular causal laws are themselves
necessary. Indeed, as far as particular causal laws are concerned, the
Second Analogy is in basic agreement with Hume: they (as synthetic
*a posteriori*) are established by induction and by induction
 alone.[8]



It is indeed crucially important to distinguish between the general
principle of causality Kant establishes in the Second Analogy and
particular causal laws. It is equally important that particular causal
laws, for Kant, are (at least for the most part) synthetic a
posteriori rather than synthetic a priori. It does not follow,
however, that Kant agrees with Hume about the status of synthetic a
posteriori causal laws. On the contrary, Kant (as we have seen)
clearly states, in SS 29 of the *Prolegomena* (the very
passage where he gives his official "answer to Hume"),
that there is a fundamental difference between a mere "empirical
rule" (heat always follows illumination by the sun) and a
genuine objective law (the sun is through its light the cause of heat)
arrived at by adding the a priori concept of cause to the merely
inductive rule. Any law thus obtained is "necessary and
universally valid", or, as Kant also puts it, we are now in
possession of "completely and thus necessarily valid
rules". In such cases (A91/B124): 



> 
> 
> The succession is *necessary*; ... the effect does not
> merely follow upon the cause but is posited *through* it and
> follows *from* it. The strict universality of the rule is
> certainly not a property of empirical rules, which, through induction,
> can acquire nothing but comparative universality: i.e., extensive
> utility. 
> 
> 
> 



Therefore, it is by no means the case that Kant simply agrees with
Hume that particular causal laws are grounded solely on induction and,
accordingly, that the necessity we attribute to particular causal
connections is merely subjective.



Similarly, the text of the Second Analogy is also committed to the
necessity and strict universality of particular causal laws. If the
general causal principle (that every event *b* must have a
cause *a*) is true, then, according to Kant, there must also be
particular causal laws (relating preceding events of type *A*
to succeeding events of type *B*) which are themselves strictly
universal and
 necessary.[9]
 Kant maintains that, when one event follows another in virtue of a
causal relation, it must always follow "in accordance with a
rule" (A193/B238). Moreover, the "rule" to which
Kant is here referring is not the general causal principle, but rather
a particular law connecting a given cause to a given effect which is
itself strictly universal and necessary (A193/B238-239): 



> 
> 
> In accordance with such a rule, there must thus lie in that which
> precedes an event in general the condition for a rule according to which
> this event follows always and necessarily. 
> 
> 
> 



Kant insists on this point throughout the Second Analogy: 



> 
> 
> that which follows or happens must follow according to a universal
> rule from that which was contained in the previous state,
> (A200/B245)
> 
> 
> 
> in that which precedes the condition is to be met with under which the
> event always (i.e., necessarily) follows, (A200/B246) 
> 
> 
> 



and so on. One cannot escape the burden of explaining the apparently
paradoxical necessity and universal validity of particular (synthetic)
*a posteriori* causal laws simply by distinguishing them from
the general (synthetic) *a priori* causal principle.



What is the relationship, then, between the general causal principle
of the Second Analogy and the particular causal laws whose existence,
according to Kant, is required by the causal principle? What, more
generally, is the relationship between the transcendental synthetic a
priori principles of the understanding (including all three Analogies
of Experience--compare the end of
 note 3
 above--as well as the principles corresponding to the other
categories) and the more particular synthetic a posteriori laws of
nature involved in specific causal relationships governing empirically
characterized events and processes? The relationship cannot be
deductive; for, if one could deductively derive the particular causal
laws from the transcendental principles of the understanding, then the
former would have to be synthetic a priori as well.



Kant himself discusses this relationship extensively, beginning in the
first edition version of the Transcendental Deduction
(A126-128):



> 
> 
> Although we learn many laws through experience, these are still only
> particular determinations of yet higher laws, among which the highest
> (under which all others stand) originate a priori in the understanding
> itself, and are not borrowed from experience, but must rather provide
> appearances with their law-governedness, and precisely thereby make
> experience possible ... To be sure, empirical laws as such can in
> no way derive their origin from pure understanding--no more than
> the immeasurable manifold of appearances can be sufficiently
> comprehended from the pure form of sensibility. But all empirical laws
> are only particular determinations of the pure laws of the
> understanding, under which and in accordance with the norm of which
> they first become possible, and the appearances take on a lawful
> form--just as all appearances, notwithstanding the diversity of
> their empirical form, still must also always be in accordance with the
> condition of the pure form of sensibility [i.e., space and time].
> 
> 
> 



The "pure laws of the understanding" (here and elsewhere)
refers to the pure transcendental *principles* of the
understanding characterizing what Kant calls "experience in
general" or "nature in general".



In the second edition version Kant makes essentially the same point,
this time explicitly stating that the relationship in question is not
deductive (B165):



> 
> 
> The pure faculty of understanding, however, is not sufficient for
> prescribing to appearances a priori, through mere categories, any laws
> other than those which are involved in a *nature in general*,
> as the law-governedness of all appearances in space and time.
> Particular laws, because they concern empirically determined
> appearances, can *not be completely derived* therefrom,
> although they one and all stand under them. Experience must be added
> in order to become acquainted with the [particular laws] *as
> such*, but only the former laws provide a priori instruction
> concerning experience in general, and [concerning] that which can be
> cognized as an object of experience.
> 
> 
> 



But what exactly does it mean for particular laws of nature to
"stand under" the a priori principles of the
understanding--that is, to be what Kant calls "particular
determinations" of these principles? Once again, it will take
more work fully to clarify this relationship, but we can meanwhile
observe that it is precisely in virtue of the relationship in question
that empirical causal connections--empirical causal laws of
nature--count as *necessary* for Kant.



The necessity in question is characterized in Kant's official
discussion of the *category* of necessity in the Postulates of
Empirical Thought--the three principles corresponding to the
categories of possibility, actuality, and necessity
(A218-218/B265-266):


1. That which agrees with the formal conditions of experience
(according to intuition and concepts), is *possible*.
2. That which coheres with the material conditions of experience
(with sensation), is *actual*.
3. That whose coherence with the actual is determined in accordance
with the general conditions of experience, is (exists as)
*necessary*.



The "formal [or "general"] conditions of
experience" include the forms of intuition (space and time),
together with all the categories and principles of the understanding.
The material conditions of experience include that which is given to
us, through sensation, in perception. Kant is thus describing a
three-stage procedure, in which we begin with the formal a priori
conditions of the *possibility* of experience in general,
perceive various *actual* events and processes by means of
sensation, and then assemble these events and processes
together--via *necessary* connections--by means of
the general conditions of the possibility of experience with which we
began.



In his detailed discussion of the third Postulate Kant makes it clear
that he is referring, more specifically, to *causal* necessity,
and to particular (empirical) *causal laws*
(A226-8/B279-80): 



> 
> 
> Finally, as far as the *third* Postulate is concerned, it
> pertains to material necessity in existence, and not the merely formal
> and logical necessity in the connection of concepts. ... Now
> there is no existence that could be cognized as necessary under the
> condition of other given appearances except the existence of effects
> from given causes in accordance with laws of causality. Thus, it is
> not the existence of things (substances), but only that of their
> state, about which we can cognize their necessity--and, indeed,
> from other states that are given in perception, in accordance with
> empirical laws of causality. 
> 
> 
> 



Note that, in this passage, Kant refers to "laws of
causality" (in the plural) in the second quoted sentence, and
"empirical laws of causality" (again in the plural) in the
last sentence. Hence, he is here referring to particular causal laws
(of the form every event of type *A* must always be followed by
an event of type *B*) rather than the general principle of the
Second Analogy (that every event *b* must have a cause
 *a*).[10]



In the Transcendental Deduction (as we have seen) Kant says that 



> 
> 
> all empirical laws are only particular determinations of the pure laws
> of the understanding, under which and in accordance with the norm of
> which they first become possible, and the appearances take on a lawful
> form. (A127-128) 
> 
> 
> 



In the discussion of the third Postulate Kant says that we can cognize
an effect as *necessary* on the basis of an empirical law
relating it to its cause--where the effect's
"connection with the actual is determined in accordance with the
general conditions of experience" (A218/B266). Kant is
suggesting, therefore, that the precise sense in which particular
empirical laws themselves become necessary is that they, too, are
"determined" in relation to actual perceptions "in
accordance with the general conditions of experience" (where the
latter, of course, essentially include the "pure laws of the
understanding", i.e., the principles).



Thus, in the example from SS 29 of the *Prolegomena*, Kant
begins from a mere "empirical rule" (that heat always
follows illumination by the sun) and then proceeds to a
"necessary and universally valid" law by adding the a
priori concept of cause to this (so far) merely inductive rule. The
very same three-stage procedure described by the three Postulates as a
whole--in which we begin with the formal a priori conditions of
the *possibility* of experience in general, perceive various
*actual* events and processes by means of sensation, and then
assemble these events and processes together (via *necessary*
connections) by means of the a priori conditions of the possibility of
experience--also results in "necessary and universally
valid" empirical causal laws of nature (the sun is through its
light the cause of heat) governing the events and processes in
question.


## 3. Kant, Hume, and the Newtonian Science of Nature



In SS 36 of the *Prolegomena* (after he has presented his
official "answer to Hume" in SS 29) Kant addresses the
question of the relationship between particular empirical laws and the
a priori principles of the understanding under the title "How is
nature itself possible?" Nature in the *material* sense
is "the totality of all appearances" given in space and
time (4, 318; 69). Nature in the *formal* sense is "the
totality of rules under which all appearances must stand if they are
to be thought as connected in an experience" (4, 318; 70). In
answering the question of how nature in the formal sense is possible
Kant proceeds to distinguish between "empirical laws of nature,
which always presuppose particular perceptions" and 



> 
> 
> the pure or universal laws of nature, which, without having a basis in
> particular perceptions, contain merely the conditions of their
> necessary unification in an experience. (4, 320; 71)
> 
> 
> 



Yet (as we have seen) the empirical laws owe their status as
"necessary and universally valid" to their relationship
with the a priori "pure or universal" laws (principles) of
the understanding. Moreover, Kant illustrates this situation with an
example, which (as explained in the very brief SS 37) 



> 
> 
> is to show, that laws that we discover in objects of sensible
> intuition, especially if they are cognized as necessary, are already
> taken by us to be such as the understanding has put there, even though
> they are otherwise similar in all respects to laws of nature that we
> attribute to experience. (4, 320; 72) 
> 
> 
> 



The example (presented in the immediately following SS 38) is a




> 
> 
> physical law of mutual attraction, extending over the whole of
> material nature, whose rule is that it diminishes inversely with the
> square of the distances from every attracting point. (4, 321; 73) 
> 
> 
> 



Thus, Kant illustrates his conception of the relationship between
particular empirical laws and the a priori principles of the
understanding with the Newtonian law of universal
 gravitation.[11]



In SS VI of the Introduction to the second edition of the
*Critique*, where Kant discusses the "general problem of
pure reason" ("How are synthetic a priori judgments
possible?"), Kant explains that 



> 
> 
> in the solution of [this] problem there is also conceived, at the same
> time, the possibility of the pure employment of reason in grounding
> and developing all sciences that contain a theoretical a priori
> cognition of objects, i.e., the answer to the questions: **How
> is pure mathematics possible? How is pure natural science
> possible?**. (B20) 
> 
> 
> 



Kant illustrates his contention that propositions of "pure
natural science" actually exist in a footnote (ibid.): 



> 
> 
> One need only attend to the various propositions that appear at the
> beginning of proper (empirical) physics, such as those of the
> permanence of the same quantity of matter, of inertia, of the equality
> of action and reaction, and so on, in order to be soon convinced that
> they constitute a pure (or rational) physics, which well deserves, as
> a science of its own, to be isolated and established in its entire
> extent, be it narrow or wide.
> 
> 
> 



Kant had just completed the latter task, in fact, in his
*Metaphysical Foundations of Natural Science*, which had
meanwhile appeared in 1786 (following the publication of the
*Prolegomena* in 1783 and immediately preceding the publication
of the second edition of the *Critique* in 1787). There Kant
articulates what he calls "pure natural science" in four
chapters corresponding, respectively, to the four headings of the
table of categories (quantity, quality, relation, and modality). In
the third chapter or Mechanics (corresponding to the three categories
of relation: substance, causality, and community) Kant derives three
"laws of mechanics" corresponding, respectively, to the
three Analogies of Experience: the permanence or conservation of the
total quantity of matter, the law of inertia, and the equality of
action and reaction--which Kant describes as a law of "the
communication of motion" (4, 544; 84). All these laws, Kant
makes clear, are synthetic a priori propositions, demonstrated a
priori and "drawn from the essence of the thinking faculty
itself" (4, 472; 8).



For Kant, therefore, the laws of the Newtonian science of nature are
of two essentially different kinds. Kant regards Newton's three
"Axioms or Laws of Motion" presented at the beginning of
the *Principia* as synthetic a priori truths--which Kant
himself attempts to demonstrate a priori in the *Metaphysical
 Foundations*.[12]
 By contrast, Kant does not regard the inverse-square law of universal
gravitation, which Newton establishes by a famous "deduction
from the phenomena" in Book 3 of the *Principia*, as a
synthetic a priori truth--and, accordingly, Kant does not attempt
to demonstrate this law a priori in the *Metaphysical
Foundations*. Nevertheless, Kant regards the synthetic *a
posteriori* law of universal gravitation as "necessary and
universally valid" in virtue of the way in which it is
"determined" in relation to the "phenomena" by
the synthetic a priori laws of pure natural science. And, since the
latter, in turn, are "determined" from the a priori
principles of the understanding, the a posteriori law of universal
gravitation is thereby "determined" in relation to actual
perceptions "in accordance with the general conditions of
 experience".[13]



We shall return to Kant's conception of Newtonian natural
science below, but we first want to discuss Hume's rather
different debt to Newton. Hume, like virtually everyone else in the
eighteenth century (including Kant), takes Newtonian natural science
as his model, and, indeed, he attempts to develop his own
"science of human nature" following Newton's
example. Yet Hume learns a very different lesson from Newton than does
Kant, based on Newtonian inductivism rather than Newtonian
mathematical demonstrations. Contrasting Hume and Kant on this point
greatly illuminates their diverging conceptions of causation and
necessity.



To begin with, Hume does not consider Newton's "Axioms or
Laws of Motion" as a priori in any sense (in Kant's
terminology, neither analytic nor synthetic a priori). All of these
laws, according to Hume, are simply "facts" inductively
derived from (constant and regular) experience. Hume considers
Newton's second law of motion (F = ma) in the *Enquiry*,
section 4, part 1 (EHU 4.13; SBN 31): 



> 
> 
> Thus, it is a law of motion, discovered by experience, that the moment
> or force of any body in motion is in the compound ratio or proportion
> of its solid contents and its velocity ... . Geometry assists us
> in the application of this law ... ; but still the discovery of
> the law itself is owing merely to experience, and all the abstract
> reasonings in the world could never lead us one step towards the
> knowledge of it.
> 
> 
> 



One of Newton's main examples of the third law of motion is the
communication of motion by impact or
 impulse.[14]
 Hume considers such communication of motion in the same section of
the *Enquiry* (EHU 4.8; SBN 28-29): 



> 
> 
> We are apt to imagine, that we could discover these effects by the
> mere operation of our reason, without experience. We fancy, that were
> we brought, on a sudden, into this world, we would at first have
> inferred, that one billiard ball would communicate motion to another
> upon impulse; and that we needed not to have waited for the event, in
> order to pronounce with certainty concerning it. Such is the influence
> of custom, that, where it is strongest, it not only covers our natural
> ignorance, but even conceals itself, and seems not to take place,
> merely because it is found in the highest degree.
> 
> 
> 



Finally, in a footnote at the end of part 1 of section 7 (the section
in the *Enquiry* devoted to the idea of necessary connection),
Hume considers the law of inertia (EHU 7.25n16; SBN 73n1): 



> 
> 
> I need not examine at length the *vis inertiae* which is so
> much talked of in the new philosophy, and which is ascribed to matter.
> We find by experience, that a body at rest or in motion continues for
> ever in its present state, till put from it by some new cause; and
> that a body impelled takes as much motion from the impelling body as
> it acquires itself. These are facts. When we call this a *vis
> inertiae*, we only mark these facts, without pretending to have
> any idea of the inert power. 
> 
> 
> 



(Hume here puts the law of inertia and the communication of motion by
impulse together, because both are consequences of a body's
"inherent force [*vis insita*]" or "inert
force [*vis inertiae*"] according to Newton's third
definition preceding the Laws of
 Motion.[15])
 It is clear, therefore, that Hume views all of Newton's laws of
motion as inductively derived empirical propositions, which
(deceptively) appear to be derived from reason simply because the
constant and regular experience on which they are in fact based is so
pervasive.



We believe that Hume's discussion of the communication of motion
by contact or impulse shows his debt to Newton especially clearly. In
section 7, part 1 of the *Enquiry* Hume is criticizing the
inherited ideas of necessary connection. We believe that both here and
in section 4, part 1, where he rejects any a priori demonstration of
causality, Hume is centrally concerned with the conception of
necessary connection articulated by the mechanical natural philosophy.
This philosophy had taken the communication of motion by contact or
impulse as the paradigm of an a priori rationally intelligible causal
connection, to which all other instances of causal connection must be
reduced. The reduction would take place by reducing all observable
causal relationships to the motions and impacts of the tiny
microscopic parts of
 bodies.[16]



In the view of contemporary mechanical philosophers, especially
Huygens and Leibniz, Newton's conception of universal
gravitation involved an entirely unintelligible action at a distance
across empty space. Gravitation could only be acceptable, on their
view, if it were explained, in turn, by vortices of intervening
invisible matter whose tiny microscopic particles effected the
apparent attraction of bodies via impulse. Although both Leibniz and
Huygens accepted Newton's demonstration that the orbits of the
satellites of the major astronomical bodies in the solar system obey
the inverse-square law (the planets with respect to the sun, the moons
of Jupiter and Saturn with respect to their planets, the earth's
moon with respect to the earth), they rejected Newton's
unrestricted generalization of this law to hold between all bodies
(and all parts of bodies) whatsoever. For them, the inverse-square law
could be accepted in astronomy only by taking the major bodies of the
solar system as each being surrounded by vortices limited to the
finite surrounding region of their satellites. The validity of the
inverse-square law would thus be restricted to precisely such a finite
region, so that it could not be extended arbitrarily far: the moons of
Jupiter would accelerate towards Jupiter, for example, but neither
Saturn nor the sun, for example, would experience such accelerations
towards
 Jupiter.[17]



In the second (1713) edition of the *Principia*, in response to
these doubts about the law of universal gravitation raised by
mechanical philosophers, Newton adds an explicit principle of
unrestricted inductive generalization--Rule 3--to a set of
"Rules for the Study of Natural Philosophy" at the
beginning of Book 3. Rule 3 states (*Principia*, 795): 



> 
> 
> Those qualities of bodies that cannot be intended and remitted [i.e.
> qualities that cannot be increased and diminished] and that belong to
> all bodies on which experiments can be made should be taken as
> qualities of all bodies
>  universally.[18]
> 
> 
> 
> 



Then, in the explanation of this Rule, Newton depicts the hypotheses
of the mechanical philosophy as in conflict with the method of
inductive generalization that leads to the law of universal
gravitation (*Principia*, 795-796): 



> 
> 
> For the qualities of bodies can be known only through experiments; and
> therefore qualities that square with experiments universally are to be
> regarded as universal qualities .... Certainly idle fancies ought
> not to be fabricated recklessly against the evidence of experiments,
> nor should we depart from the analogy of nature, since nature is
> always simple and ever consonant with itself.
> 
> 
> 



That the "idle fancies" in question include the hypotheses
of the mechanical philosophers (such as the vortex hypothesis) is made
perfectly clear and explicit in the passage from the General Scholium
(also added to the second edition in 1713) where Newton famously says
that he "feigns" no hypotheses (*Principia*, 943):




> 
> 
> I have not as yet been able to deduce from phenomena the reason for
> [the] properties of gravity, and I do not feign hypotheses. For
> whatever is not deduced from the phenomena must be called a
> hypothesis; and hypotheses, whether metaphysical or physical, or based
> on occult qualities, or mechanical, have no place in experimental
> philosophy. In this experimental philosophy, propositions are deduced
> from the phenomena and are made general by induction. The
> impenetrability, mobility, and impetus of bodies, and the laws of
> motion and the law of gravitation have been found by this
>  method.[19]
> 
> 
> 
> 



Thus, Newton also makes it clear that gravity is (at least) as well
grounded by induction as the favored properties of bodies singled out
by the mechanical philosophers (impenetrability, motion, and impetus),
*all* of which have been derived inductively from phenomena (a
point he had earlier developed in the explanation of Rule
 3).[20]



Hume (as we have seen) considers all the laws of
motion--including the communication of motion by contact or
impulse--as (merely) inductively derived general principles.
Accordingly, Hume also unreservedly accepts universal gravitation and
takes Newton's theory to articulate a fundamental law of nature
completely on a par with all other inductively established laws (EHU
6.4; SBN 57): 



> 
> 
> There are some causes, which are entirely uniform and constant in
> producing a particular effect; and no instance has ever yet been found
> of any failure or irregularity in their operation. Fire has always
> burned, and water suffocated every human creature: The production of
> motion by impulse and gravity is an universal law, which has hitherto
> admitted of no exception. 
> 
> 
> 



For Hume, contrary to the mechanical philosophy, there is no asymmetry
between the law of universal gravitation and the laws of impact with
respect to their intrinsic
 intelligibility.[21]



There is an even more fundamental relationship between Hume's
conception of the inductive method and Newton's Rule 3. In the
explanation of this Rule (as we have seen) Newton takes the
supposition that "nature is always simple and ever consonant
with itself" to license the inductive generalizations made in
accordance with the Rule. Similarly, Hume appeals, in the
*Enquiry*, to the supposition that "the course of
nature" does not change (EHU 4.21; SBN 37-38) or,
equivalently, that "the future will be conformable to the
past" (EHU 4.19; SBN 35-36). In the *Treatise* Hume
formulates this supposition as the 



> 
> 
> principle, *that instances, of which we have had no experience,
> must resemble those, of which we have had experience, and that the
> course of nature continues always uniformly the same*. (T 1.3.6.4;
> SBN 89) 
> 
> 
> 



Hume takes this supposition to license (in his own words, to provide
the "foundation" for: compare
 note 5
 above) all inductive inferences from observed constant conjunctions,
just as Newton takes the supposition that "nature is always
simple and ever consonant with itself" to license the
applications of his Rule 3. It appears very likely, therefore, that
Hume takes this Newtonian supposition as the model for his own
principle of the uniformity of
 nature.[22]



Yet Hume raises radical skeptical doubts about this very principle. It
has no foundation in reasoning: neither in demonstrative reasoning nor
(on pain of circularity) in inductive reasoning itself. Nevertheless,
as firmly based in custom or habit, it is a universal principle of the
human mind. Moreover, it is also the foundation for the best available
science of matters of fact--Newtonian inductive science--and
for Hume's own inductive science (self-consciously following
Newton) of human
 nature.[23]
 Thus, when Hume sets his radical skeptical doubts aside, the
application of our foremost empirical scientific method (based on
uniform constant conjunction) has normative force, and it thereby
leads to the articulation of universal, exceptionless laws of nature
which, as such, we are compelled to treat as necessary until
experience teaches us otherwise (in accordance with Newton's
Rule 4 in Book 3 of the *Principia*: see
 note 19
 above).[24] It is because the idea of necessary
connection, for Hume, arises from the application of the Newtonian
inductive method that our projection of an inner feeling of
determination onto nature does not merely reduce to a blind
instinctual disposition, but amounts to a normative methodological
standard in our best scientific understanding of
 nature.[25]



In the famous *hypothesis non fingo* passage from the General
Scholium Newton characterizes his "experimental" method as
follows (*Principia*, 943): "In this experimental
philosophy, propositions are deduced from the phenomena and are made
general by induction". Hume focusses exclusively on the second,
inductive, clause, and he thereby shows an especially deep insight
into the fundamental difference between Newton's methodology and
the purely demonstrative ideal of scientific knowledge represented by
the mechanical
 philosophy.[26]
 For Kant, by contrast, the dispute between Newton and the mechanical
philosophers is now effectively over; and Kant concentrates instead on
Newtonian mathematical demonstrations and the idea of "deduction
from phenomena". This comes out especially clearly in the
*Metaphysical Foundations of Natural Science*, where Kant
engages with some of the most important details of Newton's
demonstration of the law of universal gravitation from the initial
"phenomena" described at the beginning of Book 3 of the
*Principia*. Kant shows especially deep insight into the way in
which this argument is inextricably entangled, in turn, with the
Newtonian mathematical conception of (absolute) space, time, and
motion; and he thereby takes special pains to frame the explicitly
inductive steps in Newton's argument within the a priori
"special metaphysics" of nature expounded in the
*Metaphysical
 Foundations*.[27]



The "phenomena" with which Book 3 of the
*Principia* begins record the observed relative motions of the
principal satellites in the solar system with respect to their primary
bodies (the planets with respect to the sun, the moons of Jupiter and
Saturn with respect to their planets, the earth's moon with
respect to the earth). All of these satellites obey Kepler's
laws (at the time often called "rules") of orbital motion;
and, appealing to his first law of motion (the law of inertia), Newton
is able to derive purely mathematically that each of the satellites in
question experiences an inverse-square acceleration directed towards
its respective primary body. Moreover, the so-called "moon
test" (developed in Proposition 4 of Book 3) shows that the
inverse-square acceleration governing the moon's orbit is, when
the distance in question approaches the surface of the earth,
numerically equal to the constant acceleration of terrestrial gravity
figuring in Galileo's law of fall. Newton concludes (by the
first and second of his Rules for the Study of Natural Philosophy)
that the (centripetal) force holding the moon in its orbit is the same
force as terrestrial gravity.



The crucial inductive steps come next. Newton generalizes the result
of the moon test to all the other satellites in the solar system:
they, too, are held in their orbits by the same force of gravity
(Proposition 5). Then (in Proposition 6) Newton concludes that all
bodies whatsoever gravitate towards every primary body (including both
Saturn and the sun towards Jupiter, for example); moreover, their
weights, like those of terrestrial bodies, are proportional to their
masses at equal distances from the primary body in
 question.[28]
 Finally (in Proposition 7), Newton applies the third law of motion to
this last result to derive the law of universal gravitation itself:
not only do all bodies whatsoever experience inverse-square
accelerations (proportional to mass) towards every primary body in the
solar system, but the primary bodies themselves experience
inverse-square accelerations (proportional to mass) towards every
other body (Jupiter towards its moons and all other planets, the earth
towards its moon and all other planets, and so
 on).[29]
 Indeed, Newton here extends this universal conclusion to the
*parts* of all bodies as
 well.[30]



Kant accepts Newton's law of gravitation in its full universal
form--as a 



> 
> 
> physical law of mutual attraction, extending over the whole of
> material nature, whose rule is that it diminishes inversely with the
> square of the distances from every attracting point.
> (*Prolegomena*, SS 38: 4, 321; 73) 
> 
> 
> 



Moreover, Kant has no qualms at all about action at a distance, and he
even attempts to demonstrate a priori (in the *Metaphysical
Foundations*) that universal gravitation, as a manifestation of
what he calls the "original" or "fundamental"
force of attraction, *must* be conceived as an immediate action
at a distance through empty
 space.[31]
 Kant also attempts to demonstrate his three "laws of
mechanics" corresponding to Newton's three laws of motion
as synthetic a priori truths, especially the crucially important third
law (the equality of action and
 reaction).[32]
 Whereas Newton had devoted considerable effort to producing
experimental evidence for this law (see
 note 14
 above), Kant here ventures a rare criticism of Newton for not having
the courage to prove it a
 priori.[33]
 Indeed, regarding this particular law as a synthetic a priori truth
is central to Kant's reinterpretation of the Newtonian concepts
of (absolute) space, time, and motion; for it is in virtue of his
understanding of the equality of action and reaction that Kant is now
able simply to *define* the center of gravity of the solar
system (in which this principle necessarily holds) as an empirically
determinable (provisional) surrogate for Newtonian absolute
 space.[34]
 Moreover, and for closely related reasons, Kant takes the
universality of what he calls the "original" or
"fundamental" force of attraction--that it proceeds
from every part of matter to every other part to infinity--as
another synthetic a priori truth demonstrable in "pure natural
 science".[35]



Given this foundation in "pure natural science", Kant then
reconstructs Newton's "deduction from the phenomena"
of the law of universal gravitation as follows. We begin, following
Newton, from the observable "phenomena" described by
Kepler's "rules". These "phenomena", in
Kant's terminology, are so far mere "appearances
[*Erscheinungen*]", which have not yet attained the
status of "experience
 [*Erfahrung*]".[36]
 Then, again simply following Newton, we can use the law of inertia to
derive (purely mathematically) inverse-square accelerations of their
satellites directed towards every primary body in the solar system.
Once we have done this, however, we can now, from Kant's point
of view, frame all of Newton's explicitly inductive steps within
the a priori "special metaphysics" of nature developed in
the *Metaphysical Foundations*. By demonstrating a priori his
three "laws of mechanics" corresponding to the three
Analogies of Experience, Kant establishes that Newton's three
"Axioms or Laws of Motion" are synthetic a priori truths
(compare notes
 12
 and
 32
 above). Further, by identifying the accelerations in question as
effects of what Kant calls the fundamental force of attraction, it now
follows from Kant's "special metaphysics" of
(material) nature that these accelerations must hold immediately
between each part of matter and every other part of matter--and,
accordingly, are also directly proportional to the
 mass.[37]



In the fourth chapter or Phenomenology of the *Metaphysical
Foundations* Kant connects this reconstruction of Newton's
argument with the modal categories of possibility, actuality, and
necessity--the very categories which (as we saw at the end of the
second section above) make it possible for initially merely inductive
generalizations (a la Hume) to acquire the status of necessary
laws. The first stage, where we simply record the
"phenomena" described by Kepler's
"rules" (as mere "appearances":
 note 36
 above), corresponds to the category of possibility. The second stage,
where we say that we here have instances of "true" (as
opposed to merely "apparent") rotation by appealing to the
law of inertia, corresponds to the category of
 actuality.[38]
 In the third stage, finally, we apply the equality of action and
reaction to the true centripetal accelerations correlated with such
true rotations
 (note 38
 above); and all of them, in accordance with Kant's metaphysical
"dynamical theory of matter", must now be taken as
extending universally to infinity from each attracting point (compare
notes
 35
 and
 37
 above). The result is the law of universal gravitation, now seen as
falling under the category of necessity. In this way, Kant's
reconstruction of Newton's "deduction" of the law of
universal gravitation from the initial Keplerian
"phenomena" provides a perfect illustration of the
three-step procedure, described in the Postulates of Empirical
Thought, by which a mere "empirical rule" is transformed
into a "necessary and universally valid" objective
 law.[39]


## 4. Time Determination, the Analogies of Experience, and the Unity of Nature



We have suggested that Kant's reconstruction of Newton's
"deduction from the phenomena" of the law of universal
gravitation in the *Metaphysical Foundations of Natural
Science* is inextricably entangled with his reinterpretation of
the Newtonian concepts of (absolute) space, time, and
 motion.[40]
 Indeed, Kant begins the *Metaphysical Foundations* by defining
matter as "the movable in space"--and by introducing
a distinction between absolute and relative space which is clearly
derived from Newton's Scholium on space, time, and motion at the
beginning of the *Principia* (see
 note 38
 above). In Newton's words (*Principia*, 408-409):




> 
> 
> Absolute space, of its own nature without reference to anything
> external, always remains homogeneous and immovable. Relative space is
> any movable measure or dimension of this absolute space. 
> 
> 
> 



In Kant's words (4, 480; 15): 



> 
> 
> *Matter* is the *movable* in space. That space which is
> itself movable is called material, or also *relative space*.
> That space in which all *motion* must finally be thought (and
> which is therefore itself absolutely immovable) is called pure, or
> also *absolute space*.
> 
> 
> 



It turns out, however, that Kant's own view, in apparent
contrast with Newton's, is that "absolute space is *in
itself* nothing and no object at all", but signifies only an
indefinite process of considering ever more extended relative spaces
(4, 481-482; 16-17). Moreover, when Kant returns to this
issue in the Phenomenology chapter (compare
 note 36
 above), he states that 



> 
> 
> absolute space is therefore not necessary as the concept of an actual
> object, but only as an idea, which is to serve as the rule for
> considering all motion and rest therein merely as relative. (4, 560;
> 99) 
> 
> 
> 



Kant's procedure for deriving "true motions" from
"apparent motions" does not conceive true motions as
taking place in a pre-given absolute space, but views them as the
product of an indefinitely extended process of empirical determination
taking place within experience itself: we begin from our parochial
perspective here on the surface of the earth, proceed (in accordance
with the argument of Book 3 of Newton's *Principia*) to
the center of gravity of the solar system, then proceed to the center
of gravity of the Milky Way galaxy, and so on *ad
 infinitum*.[41]



Similarly, it is a central theme of the Analogies of Experience in the
first *Critique* that "absolute
time"--"time itself" (B219), "time for
itself" (B225), or "time in itself" (B233)--is
no actual object of perception. Hence, the three "modes of
time" (duration, succession, and simultaneity) must all be
determined in and through perceptible features of the appearances.
Kant calls this procedure "time determination" (more
precisely, "the determination of the existence of appearances in
time"), and he sums up his view as follows (A215/B262):



> 
> 
> These, then, are the three analogies of experience. They are nothing
> else but the principles for the determination of the existence of
> appearances in time with respect to all of its three modes, the
> relation to time itself as a magnitude (the magnitude of existence,
> i.e., duration), the relation in time as a series (successively), and
> finally [the relation] in time as a totality of all existence
> (simultaneously). This unity of time determination is thoroughly
> dynamical; that is, time is not viewed as that in which experience
> immediately determines the place of an existent, which is impossible,
> because absolute time is no object of perception by means of which
> appearances could be bound together; rather, the rule of the
> understanding, by means of which alone the existence of the
> appearances can acquire synthetic unity with respect to temporal
> relations, determines for each [appearance] its position in time, and
> thus [determines this] a priori and valid for each and every time.
> 
> 
> 



For Kant, therefore, the temporal relations of duration, succession,
and simultaneity cannot be viewed as pre-existing, as it were, in an
absolute time subsisting prior to and independently of the procedures
of our pure understanding for determining these relations within the
appearances themselves. On the contrary, temporal relations as such
are the products of an empirical construction whereby we objectively
determine the appearances as objects of a unified experience by means
of the a priori principles of the Analogies. Thus, just as Kant does
not view the determination of true motions from apparent motions as
taking place within an infinite empty absolute space, he also rejects
an analogous conception of absolute time and replaces it, too, with a
process of empirical determination taking place within experience
itself.



Indeed, there is an intimate relationship between these two procedures
for empirical determination--of time and of motion, respectively.
At the very beginning of his famous Scholium Newton distinguishes
between "true" and merely "apparent" time
(*Principia*, 408): 



> 
> 
> Absolute, true, and mathematical time, in and of itself and of its own
> nature, without reference to anything external, flows uniformly and by
> another name is called duration. Relative, apparent, and common time
> is any sensible measure (whether accurate or nonuniform) of duration
> by means of motion: such a measure--for example, an hour, a day,
> a month, a year--is commonly used instead of true time. 
> 
> 
> 



Then, several pages later, Newton illustrates the difference between
"absolute" and "relative" time with reference
to the celestial motions studied in astronomy (*Principia*,
410):



> 
> 
> In astronomy, absolute time is distinguished from relative time by the
> equation of common time. For natural days, which are commonly
> considered equal for the purpose of measuring time, are actually
> unequal. Astronomers correct this inequality in order to measure
> celestial motions on the basis of a truer time. It is possible that
> there is no uniform motion by which time may have an accurate measure.
> All motions can be accelerated and retarded, but the flow of absolute
> time cannot be changed. The duration or perseverance of the existence
> of things is the same, whether their motions are rapid or slow or
> null; accordingly, duration is rightly distinguished from its sensible
> measures and is gathered from them by means of an astronomical
> equation.
> 
> 
> 



Newton is here referring to the standard astronomical procedure,
already well-understood in ancient astronomy, whereby we correct the
ordinary measure of time in terms of days, months, and years so as to
obtain "sidereal" or mean solar time based on the motions
of the sun relative to both the earth and the fixed
 stars.[42]



In the Refutation of Idealism added to the second edition of the
*Critique* Kant argues that all empirical determination of
time--including determination of the temporal relations among
one's own inner states--ultimately depends on the
perception of outer things, and, in particular, on the perception of
motion in space (B277-278):



> 
> 
> All empirical employment of our cognitive faculties in the
> determination of time fully agrees with this. It is not only that we
> can undertake all time determination only by the change of external
> relations (motion) in relation to the permanent in space (e.g., motion
> of the sun with respect to objects on the earth), but we also have
> nothing at all permanent, which could underlie the concept of a
> substance, as intuition, except merely *matter*, and even this
> permanence is not derived from outer experience, but is rather
> presupposed a priori as necessary condition of all time determination,
> and thus also [of] the determination of inner sense with respect to
> our own existence by means of the existence of outer things.
> 
> 
> 



In emphasizing that only *matter* can instantiate the concept
of substance here, Kant is alluding to the way in which the
conservation of the total quantity of matter, in the *Metaphysical
Foundations*, realizes the (transcendental) principle of the
conservation of
 substance.[43]
 Moreover, Kant's language at B277-278 (we
"*undertake* [*vornehmen*]" time
determination by observing "motion of the sun with respect to
objects on the earth") thereby suggests a progressive empirical
procedure in which we begin with our perspective here on earth,
measure the duration of time by the apparent motion of the sun, and
then proceed to correct this measure in light of our evolving
astronomical
 knowledge.[44]



For Kant, once again, this need for correction is not an indication of
a pre-existing absolute time subsisting prior to and independently of
our empirical procedures for determining temporal magnitudes from
observable motions. It rather implies that empirically observable
motions must be subject to a priori principles of the understanding (a
priori rules of time determination) in order to count as fully
objective experience within a unified, temporally determinate
objective world. Applying the relevant principles of the
understanding--the Analogies of Experience--therefore
results in a sequence of successive corrections or refinements of our
ordinary temporal experience, as the observable motions are
progressively embedded within an increasingly precise and refined
conception of temporality itself.



In the *Metaphysical Foundations*, in particular, Kant
articulates a specific realization of the Analogies of Experience in
terms of the Newtonian theory of universal gravitation. Kant's
three "laws of mechanics" (a version of the Newtonian laws
of motion: compare notes
 12
 and
 32
 above) correspond to the three principles of the Analogies; the
categories of substance, causality, and community are realized by the
system of Newtonian massive bodies interacting with one another in the
context of what Newton, in Book III of the *Principia*, calls
the System of the World. The category of substance, that is, is
realized by the conservation of the total quantity of matter (mass) in
all interactions involving these bodies (compare
 note 43
 above, together with the sentence to which it is appended); the
category of causality is realized by the gravitational forces through
which these interactions take place (in accordance with the law of
inertia); and the category of community is realized by the
circumstance that precisely these forces are everywhere mutually
equal and opposite. The temporal relation of duration is thereby
realized by the progressive empirical procedure by which we
successively correct our ordinary measure of time in light of our
evolving astronomical knowledge (compare
 note 44
 above, together with the sentence to which it is
 appended).[45]
 The temporal relation of succession is realized by the deterministic
evolution of the motions of the bodies (masses) in question described
by the law of universal gravitation (according to which every later
state of the system is uniquely determined by its earlier
 states).[46]
 The temporal relation of simultaneity, finally, is realized by the
circumstance that gravitational forces *instantaneously*
connect each body in the system with all other
 bodies.[47]
 It is in precisely this sense that the procedure of time
determination Kant describes in the Analogies is intended to replace
the conception of a pre-given absolute time.



We have now arrived at the most fundamental divergence between Kant
and Hume concerning causation and induction. For Hume, the order of
time is empirically given by the sequence of impressions and ideas
(and associations among them) which in fact happen to appear before
the mind. As Kant explains in the Second Analogy, however, such a
sequence, from his point of view, is 



> 
> 
> merely something *subjective*, and determines no object, and
> can therefore in no way count as cognition of any object at all (not
> even in the appearance). (A195/B240) 
> 
> 
> 



For Kant, it is only the a priori concept of causality (requiring a
necessary rule of connection between preceding and succeeding events)
which can then transform a merely subjective temporal sequence into an
objective one (ibid.):



> 
> 
> If we thus experience that something happens, then we always
> presuppose thereby that something precedes on which it follows in
> accordance with a rule. For otherwise I would not say of the object
> that it follows, because the mere sequence in my apprehension, if it
> is not determined by means of a rule in relation to something
> preceding, justifies no sequence in the object. Therefore, it is
> always in reference to a rule, in accordance with which the
> appearances in their sequence (i.e., as they happen) are determined
> through the previous state, that I make my subjective synthesis (of
> apprehension) objective, and, it is solely under this presupposition
> that even the experience of something happening is possible.
> 
> 
> 



It is for precisely this reason, Kant concludes, that mere induction
alone cannot be the ground for objective causal
connections--which presuppose both strict universality and
necessity, and therefore must be grounded on a priori concepts and
principles of the pure understanding
(A195-196/B240-241):



> 
> 
> It seems, to be sure, that this contradicts all remarks that have
> always been made concerning the course of the employment of our
> understanding, according to which we have only been first guided by
> the perception and comparison of many concurring sequences of events
> following on certain appearances to discover a rule, in accordance
> with which certain events always follow on certain appearances, and we
> have thereby been first prompted to make for ourselves the concept of
> cause. On such a basis this concept would be merely empirical, and the
> rule it supplies, that everything that happens has a cause, would be
> just as contingent as experience itself: its universality and
> necessity would then be only feigned and would have no true universal
> validity, because they would not be grounded a priori but only on
> induction.
> 
> 
> 



For Kant, the concept of cause cannot possibly arise from a mere
repetition of resembling constant conjunctions ("concurring
sequences of events following on certain appearances") producing
a merely subjective
 custom.[48]
 The procedure by which we apply the concept of cause to experience
cannot be merely inductive in the Humean sense; it must rather involve
a priori rules of the understanding through which we progressively
determine the objective causal relations between appearances--and
thereby determine the objective order of succession in time
 itself.[49]



Kant thus has a completely different perspective from Hume's
concerning the uniformity of nature. For Hume, the principle of
uniformity is a supposition implicit in all of our inductive
inferences leading to the formulation of laws of nature. If this
principle itself had a foundation in the understanding (in either a
priori or a posteriori "reasoning"), then so would our
inductive inferences from observed constant conjunctions to so far
unobserved events. Yet the supposition in
question--"*that instances, of which we have had no
experience, must resemble those, of which we have had experience, and
that the course of nature continues always uniformly the
same*" (T 1.3.6.4; SBN 89)--cannot itself be justified
by either demonstrative or inductive reasoning. In the former case it
would have to be self-contradictory to imagine that the course of
nature is not sufficiently uniform; in the latter the attempted
justification would be viciously circular. The principle of
uniformity, however, is firmly based in custom or habit, as a
universal principle of the human mind, and it is also the foundation
for the Newtonian inductive method--including Hume's own
inductive science of the human mind. Although the principle thus has
normative force in all our reasoning concerning matters of fact in
both science and common life, it cannot ultimately legitimate the
attribution of objective necessity to our inductively established laws
of
 nature.[50]



Kant, in our view, is attempting to provide precisely such a grounding
of objective necessity by means of the general principle of the
Analogies of Experience (B218): "Experience is possible only by
means of the representation of a necessary connection of
perceptions". More specifically, the Analogies of Experience
provide an a priori conception of the unity and uniformity of
experience playing the role, for Kant, of Hume's principle of
the uniformity of nature. According to the Analogies we know a priori
that nature in general must consist of interacting substances in space
and time governed by universally valid and necessary causal laws
determining the temporal relations (of duration, succession, and
simultaneity) among all empirical events, and this articulated a
priori conception of nature in general amounts to the knowledge that
nature is, in fact, sufficiently
 uniform.[51]



We can only have objective experience of particular events, for Kant,
in so far as we simultaneously construct particular causal relations
among them step by step, and this is only possible, in turn, in so far
as we presuppose that they are one and all parts of a unified and
uniform experience of nature in space and time governed by the
Analogies of Experience (together with the other principles of pure
understanding). Moreover, since particular causal relations, for Kant,
necessarily involve causal laws, all of our inferences from particular
perceptions to universal causal laws of nature are grounded in
synthetic a priori principles of pure understanding providing a
synthetic a priori conception of the unity and uniformity of nature in
general. Hume was correct, therefore, that the principle of the
uniformity of nature governs all of our inductive causal inferences;
and he was also correct that this principle is not and cannot be
*analytic* a priori. What Hume did not see, from Kant's
point of view, is that the merely comparative universality of
inductive generalization can indeed be overcome by transforming
initially merely subjective "empirical rules" into truly
objective and necessary "universal laws" in accordance
with *synthetic* but still a priori principles of the unity of
nature in
 general.[52]