## 1. Physics: The Pre-Critical Period



Kant's early pre-Critical publications aim to solve a variety of
broadly cosmological problems, and to develop an increasingly
comprehensive metaphysics that would ground the matter theory required
by the solutions to these problems. Kant's first publication,
*Thoughts on the True Estimation of Living Forces* (1746),
explicitly attempts to solve the *vis viva* controversy, which
had been hotly contested ever since Leibniz's 1686 attack on
Descartes' laws of motion. While Kant attempts to occupy an
intermediary position between the Cartesian and Leibnizian positions
by maintaining that both *mv* and *mv*2 could be
conserved in different contexts, what is of particular note is how his
solution in Parts II and III rests on the conception of force
developed in Part I. According to this conception, force is understood
in terms of the activity of substances, an activity that Kant then
uses to explain how the motions of bodies are generated, to solve the
mind-body problem, and to account for both the possibility of other,
actually existing worlds and the three-dimensionality of space.



His solution to the *vis viva* dispute is especially
interesting, because it presages his later approach to philosophical
controversy. Rather than offering a conclusive argument in favor of
one position, Kant seeks to mediate between the two parties,
Leibnizian and Cartesian. He argues that each measure of force is
correct, but in different contexts. Kant distinguishes two ways of
studying bodies, "mathematical" and
"metaphysical," and claims that they presuppose distinct
conceptions of body. According to mathematical mechanics, a body
cannot accelerate unless an external cause acts on it; for that
reason, Kant declares, the Cartesians' quantity
*m**v*** is the only one appropriate measure of
force in this context. "Natural bodies," in contrast, have
features that mathematics brackets. One such feature is a capacity for
"vivification," whereby a body increases by itself the
force of motion that an external cause merely "awakens."
In light of that, Kant concludes, the Leibnizian quantity
*mv*2 is the correct measure of force in
metaphysical considerations of "natural bodies"
(1:140f).



Kant develops his account of the nature of substance in greater detail
in *A New Elucidation* *of the First Principles of
Metaphysical Cognition* (1755). While the first two sections of
this work undertake revisions of Wolff's principles of
non-contradiction and sufficient reason, the third section argues for
two substantive principles that are alleged to follow from the
principle of sufficient (or rather, following Crusius, determining)
reason, namely the principles of succession and coexistence. The main
thrust of the principle of succession is directed against Leibnizian
pre-established harmony, arguing that only causal connections
*between* substances can bring about changes in their states.
Kant's position appears to be designed to account primarily for
changes of bodily states (with changes in mental states being
parasitic upon them, as was explicitly asserted in the *True
Estimation*). For he maintains that *mutual* changes of
state require *mutual* interaction, where it is clear that
changes in motion are precisely the kind of mutual change that he has
in mind (since one body cannot move closer to another without the
other body moving closer to it). The principle of coexistence then
argues that harmonious causal interaction between otherwise isolated,
independently existing substances is possible only by means of
God's coordination (just as Leibniz thought was required for
harmonious relations between the *states* of such
substances).



Kant's *Physical Monadology* (1756) articulates a theory
of matter that can reconcile the infinite divisibility of space, as
maintained in geometry, with the simplicity of substances, which Kant
believes is required in metaphysics. As was the case with his earlier
works, the essential feature of his reconciliation lies in the way in
which his matter theory is supported by his metaphysical views.
Specifically, Kant asserts that simple substances fill space not by
means of their mere existence, but rather in virtue of their spheres
of activity. As a result, any division of the relevant spheres of
activity does not compromise the simplicity of the substances
themselves, since the spatial properties of substances (including the
infinite divisibility of space) arise from the interaction between
their activities rather than from their intrinsic features. In the
course of the *Physical Monadology*, Kant also argues for the
necessity of attractive and repulsive forces and attributes a
significant role to the force of inertia. Kant's acceptance of
such Newtonian principles represents an important change of position
over the *True Estimation*, where Kant rejects the principle of
inertia and pursues a dynamical theory much more in line with
Leibniz's views.



Later in his pre-Critical period (1763-70), Kant attempts to build a
comprehensive metaphysical account on the basis of the framework that
he had established in his first works. Thus, in his *The Only
Possible Basis for a Demonstration of the Existence of God* (1763)
he attempts to extend his reasoning to fundamental issues in both
philosophical theology and teleology, presenting, for the first time,
his now famous criticisms of the three traditional arguments for the
existence of God, while developing a new theistic proof, based on the
idea that God is necessary as a real ground of the possibilities of
things. After reading Hume's *Enquiry concerning Human
Understanding* in German translation sometime after 1755, Kant
distinguishes between real and logical grounds/opposition in his
*Attempt to Introduce the Concept of Negative Magnitudes into
Philosophy* (1763) to avoid Hume's objection that there is
no logical contradiction in the existence of one thing not following
the existence of another. But in this work he is also interested in
exploring the notion of a real ground/opposition further by applying
it more widely, e.g., to bodies, mental states, etc. Also relevant is
Kant's *Concerning the Ultimate Foundation of the Distinction
of the Directions in Space* (1768) which modifies his earlier
account of space insofar as he seems to hold that certain spatial
properties -- viz., chirality, or handedness -- may not be
able to be explained entirely in terms of direct relations between
material substances. In his so-called *Inaugural Dissertation*
(1770), Kant continues to develop a more comprehensive philosophical
system, which would encompass the principles of both the sensible and
the intelligible world, and in so doing modifies his account of space
and time even further. Over the course of the next ten years, during
which he published almost nothing, Kant would revise his views more
systematically, with the publication of the *Critique of Pure
Reason* in 1781 representing the first major step in his
"critical turn."



Adickes (1924), Harman (1982), Friedman (1992), Laywine (1993),
Schonfeld (2000), Kuehn (2001), Lefevre & Wunderlich (2000),
and Watkins (1997, 2001, 2003, 2006, 2013) have emphasized the
importance of scientific issues in the development of Kant's
thought during his pre-Critical period, as he reacted to Leibniz,
Newton and other, more immediate predecessors (such as Christian
Wolff, Chr. A. Crusius, Leonhard Euler, Maupertuis, and Martin
Knutzen). Smith (2013) reconstructs Kant's picture of matter in
the *Physical Monadology*.


## 2. Physics: The Critical Period (*Metaphysical Foundations of Natural Science*)



Though Kant discusses issues relevant to physics in various works
throughout the Critical period (esp. the *Critique of Pure
Reason*), his views on this topic are developed most explicitly in
the *Metaphysical Foundations of Natural Science* (1786), which
consists of a preface and four chapters.


### 2.1 Preface



In the Preface to the *Metaphysical Foundations* Kant (i)
analyzes the concepts of nature and science so as to establish what
conditions must be met for a body of knowledge to constitute natural
science in the proper sense, (ii) explains why science so understood
requires "a pure part" (4:469) and what criteria would
have to be satisfied for such a pure part to exist, (iii) argues that
chemistry and psychology cannot at present meet these criteria, and
(iv) describes what procedure should be followed to satisfy these
criteria and thus to provide the "pure part" that science
proper requires.



The feature of Kant's conception of natural science proper that
is most immediately striking is how restrictive it is. It requires
that cognition (i) be systematically ordered (ii) according to
rational principles and (iii) be known a priori with apodictic
certainty, i.e., with "consciousness of their necessity"
(4:468). Because properly scientific cognition must satisfy these
strict conditions, it requires "a pure part on which the
apodictic certainty that reason seeks can be based" (4:469). But
since Kant identifies pure rational cognition that is generated from
concepts with metaphysics, it follows that science proper requires a
metaphysics of nature. He then specifies that such a metaphysics of
nature could consist in either a "transcendental part,"
which discusses the laws that make possible the concept of a nature in
general -- "even without relation to any determinate object
of experience" (4:469) -- or a "special
metaphysical" part, which concerns a "particular nature of
this or that kind of things" for which an empirical concept is
given.



Kant's very conception of natural science proper thus
immediately gives rise to several systematically important questions.
First, if the "transcendental part" of the metaphysics of
nature can be identified with the results of the *Critique of Pure
Reason*, then the *Metaphysical Foundations* is a work in
special metaphysics. But what exactly is a special metaphysics? In
particular, what particular natures or kinds of things could be its
object? And how precisely can an empirical concept of such things be
given without compromising the necessity required of the pure part of
natural science? Second, how is the special metaphysics provided by
the *Metaphysical Foundations* supposed to be related to the
transcendental part of the metaphysics of nature that was established
in the *Critique of Pure Reason*? Does the former presuppose
the principles of the latter or are they logically independent, but
still related to each other in some other way? Another question
concerns the method of special metaphysics. Is that method the
conceptual analysis (of the notion of matter), the transcendental
investigation of the presuppositions of the mathematical science of
nature, or something else entirely?



First, Kant suggests that in special metaphysics the principles of the
transcendental part "are applied to the two species of objects
of our senses" (4:470). Thus, the particular kinds of things
that could be investigated in a special metaphysics are (i) the
objects of outer sense, i.e., matter, and (ii) the objects of inner
sense, i.e., thinking beings, which would thus result in a doctrine of
body and a doctrine of soul. Kant then argues that because "the
possibility of determinate natural things cannot be cognized from
their mere concepts ... it is still required that the
*intuition* corresponding to the concept be given *a
priori*, that is, that the concept be constructed" (4:470),
which is a task that requires mathematics. This is Kant's
justification for his famous claim that "in any special doctrine
of nature there can be only as much *proper* science as there
is *mathematics* therein" (4:470). This argument suggests
that the necessity required of the pure part of natural science
derives from the necessity of the rules by which the mathematical
construction of determinate things must proceed.



Kant then uses the claim that science proper requires the construction
of the concept of the object in a priori intuition to exclude the
possibility that chemistry and psychology, at least as they were
practiced at that time, could count as science proper. In the case of
chemistry, the problem is that "no law of the approach or
withdrawal of the parts of matter can be specified according to which
... their motions and all the consequences thereof can be made
intuitive and presented *a priori* in space (a demand that will
only with great difficulty ever be fulfilled)" (4:471). Since
its principles are "merely empirical," it can, at best, be
a "systematic art" (ibid.). The case of psychology is more
complex, since Kant provides (at least) two separate reasons in the
Preface for denying it the status of natural science proper. First,
Kant claims that mathematics is inapplicable to the phenomena of inner
sense and their laws, though he grants that the law of continuity
(discussed, e.g., at A207-209/B253-255 and
A228-229/B281 in the *Critique of Pure Reason*) ought to
apply to changes in our representations as well. He downplays the
significance of this application of the law of continuity, however, by
noting that time has only one dimension, which does not provide enough
material to extend our cognition significantly. Second, Kant also
complains that empirical psychology cannot separate and recombine the
phenomena of inner sense at will; rather, our inner observations can
be separated "only by mere division in thought" (4:471).
Kant's fuller views on chemistry and psychology will be
discussed further below.



Second, in explaining how mathematics can be applied to bodies Kant
asserts that "principles for the *construction* of the
concepts that belong to the possibility of matter in general must
first be introduced. Therefore a complete analysis of the concept of a
matter in general [must be provided in which it] makes use of no
particular experiences, but only that which it finds in the isolated
(although intrinsically empirical) concept itself, in relation to the
pure intuitions in space and time, and in accordance with laws that
already essentially attach to the concept of nature in general"
(4:472). Kant then explains that this means that the concept of matter
must be determined according to the *Critique of Pure
Reason*'s categories of quantity, quality, relation, and
modality (4:474-476). Further, Kant holds that "a new
determination" (4:476) must be added to the concept of matter in
each chapter of the *Metaphysical Foundations*. This suggests
not only that the principles argued for in the *Metaphysical
Foundations* are to be developed "in accordance with"
the principles defended in the *Critique of Pure Reason*, but
also that both the concept of matter and the *Metaphysical
Foundations* itself is structured according to the *Critique of
Pure Reason*'s table of categories.



Unfortunately, these points of clarification do not resolve all of the
issues that are immediately raised by Kant's pronouncements
about what is required for natural science proper. One further issue
that is relevant here concerns the concept of matter that is at the
heart of the *Metaphysical Foundations*. Kant introduces it in
the *Critique of Pure Reason* (A847-848/B875-876)
as the concept of something that is impenetrable, extended, and inert.
Yet, in the beginning of the Preface of the *Metaphysical
Foundations*, he describes it as whatever is an object of outer
sense, and later he argues that the "basic determination of
something that is to be an object of the outer senses had to be
motion, because only thereby can these senses be affected"
(4:476). Whatever weight one accords Kant's justification of the
connection between matter, outer sense, and motion, one faces a
dilemma. If the concept of matter, most fundamentally, is simply the
concept of any object of outer sense, then how is it still empirical
in any genuine sense (and what has become of the structural difference
Kant draws between the *Critique of Pure Reason* and the
*Metaphysical Foundations*)? If, by contrast, impenetrability,
extension, and movability are deemed the basic traits of the concept
of matter, then how can one know a priori that any object we might
encounter in outer sense must behave in accordance with the laws that
would govern matter so defined?



Moreover, even if one can find an appropriately nuanced sense in which
the concept of matter is empirical while still allowing for an
appropriate kind of necessity, questions can still be posed about the
"new determinations" that are to be added to that concept
in each chapter of the *Metaphysical Foundations*. For example,
what is the justification for each specific determination that is
added when one thinks of matter as having a quantity, a quality, etc.?
Also, what is the relationship between each new determination of
matter and the various claims that Kant makes in each chapter of the
*Metaphysical Foundations*? In particular, when Kant explicitly
invokes principles for constructing concepts belonging to the
possibility of matter, is his idea that these principles are required
insofar as they make experience of the relevant "new
determination" of matter possible (so that Kant would be
developing a transcendental argument in the *Metaphysical
Foundations* similar in many respects to the *Critique of Pure
Reason*)? Answers to these questions depend on how one interprets
the arguments Kant develops throughout the *Metaphysical
Foundations*.



The conception of science that Kant presents in the Preface has been
the focus of considerable attention over the past several decades. In
the German literature, the issues raised above have been discussed at
length by Plaass (1965), Cramer (1985), and others. Pollok (2001) has
recently produced a detailed and comprehensive textual commentary on
the *Metaphysical Foundations*. Important work has also been
done in the English literature by Walker (1974), Brittan (1978),
Buchdahl (1968, 1969, and 1986), Parsons (1984), Butts (1986), and
Watkins (1998a). Friedman (1992, 2001, 2002 and 2013) has been
especially influential on these issues as well. Several chapters (by
Sturm, Breitenbach, and Friedman, respectively) in McNulty 2022 add to
our understanding of this aspect in Kant's thought.


### 2.2 Phoronomy



The first chapter of the *Metaphysical Foundations*, the
Phoronomy, considers the quantity of motion of matter and how it is to
be constructed in intuition a priori (so as to produce the kind of
rules that are necessary for our experience of matter in motion).
Since extension and impenetrability are not directly relevant to how
different magnitudes (or degrees) of motion can be represented, Kant
restricts his discussion in this chapter to matter considered as a
point. Since the motion of a point in space can be represented
straightforwardly, the main issue is how to represent the composition
of two different motions. Kant's primary claim in this chapter
is that due to the relativity of space (i.e., the fact that every
motion can be viewed arbitrarily as either the motion of a body in a
space at rest, or as a body in a state of rest in a space which is in
motion in the opposite direction with the same velocity) "the
composition of two motions of one and the same point can only be
thought in such a way that one of them is represented in absolute
space, and, instead of the other, a motion of the relative space with
the same speed occurring in the opposite direction is represented as
the same as the latter" (4:490). The proof of this Theorem
considers the three possible cases for the composition of two motions:
(i) The two motions are in the same direction; (ii) the two motions
are in opposite directions; (iii) the two motions enclose an angle.
Kant then shows how one can construct a priori in intuition a single
motion out of the two motions described in cases (i)-(iii). The
synthetic a priori outcome of this constructive procedure is a
composition theorem that covers two fundamental results of classical
physics: the parallelogram rule for velocity addition, and the
Galilean kinematic transformations. The theorem is needed for
architectonic reasons too, not just as a foundation for science. Kant
uses the composition theorem as a premise in his Dynamics, so as to
infer a priori to forces from the composite motions they cause (e.g.,
4:497). And, he invokes the theorem explicitly in his Mechanics, in
the course of "constructing the communication of motion,"
i.e. deriving the laws of impact (4:546).



Until recently, very little has been written directly on Kant's
Phoronomy. Pollok (2001) is a historically rich commentary. Friedman
(2013) untangles the complex strands of thought in Kant's
chapter. His leading idea is that Kant's categories of quantity
(of the First Critique) guarantee that motion is a magnitude, hence
mathematics is applicable to the motions of bodies--as
kinematics. Two chapters (by Dunlop and Stan, respectively) in McNulty
2022 add to our understanding of this aspect in Kant's
thought.


### 2.3 Dynamics



The second chapter of the *Metaphysical Foundations*, the
Dynamics, considers how it is possible to experience matter as filling
a determinate region in space. Propositions 1-4 are devoted to
exhibiting the nature and necessity of repulsive forces. In
Proposition 1 Kant argues that repulsive force is required for matter
to fill space, since solidity, understood by "Lambert and
others" as the property matter would have by means of "its
mere *existence*" (4:497), cannot truly explain how one
matter resists another matter's attempt to penetrate it. Kant
then specifies several central features of repulsive forces in
Propositions 2 and 3. Repulsive forces admit of degrees to infinity,
since one must always be able to think of a slightly greater or lesser
force, and although matter can be compressed to infinity, it can never
be penetrated, since that would require an infinite compressing force,
which is impossible.



In Proposition 4 Kant draws an important consequence from his
characterization of repulsive forces, namely that matter is infinitely
divisible (4:503). What is especially striking about this point is
that it represents a significant departure from his own earlier
*Physical Monadology*, where he had accepted attractive and
repulsive forces, but denied the infinite divisibility of what
ultimately constitutes matter, namely physical points or monads. It is
true that part of Kant's rationale for his change of position on
this point stems from the "critical turn" undertaken in
the *Critique of Pure Reason* (and in its Second Antinomy in
particular). For once one recognizes that both space and spatial
properties such as divisibility are not properties of things in
themselves but rather only appearances, one can reject the proposition
that seems to necessitate the acceptance of simple substances, namely
the idea that simple substances must precede the wholes they compose
(4:506). However, Kant's proof also seems to depend, in its
details, not merely on the idea that every space is filled by means of
*some* repulsive force or other, but on the stronger claim that
every space is divisible into smaller spaces that are filled by
*different* repulsive forces.



Propositions 5-8 are all devoted to attractive force. In
Proposition 5 Kant argues that matter must have an attractive force in
order to fill space. Kant's argument is that if there were only
repulsive forces, then matter "would disperse itself to
infinity" (4:508) since neither space nor other matter could
limit it. Proposition 6 argues that both attractive and repulsive
forces must be considered essential to matter. That is, attractive
forces alone are not sufficient to account for matter filling a space,
since if matter consisted solely of attractive forces, there would be
no force to counteract the attractive force being exercised and the
universe would collapse into a single point. Together, Propositions 5
and 6 make up a 'balancing argument', which Kant had
already used in the earlier *Physical Monadology*. (A balancing
argument is an existence proof for a type of force. Its premises are
(1) an accepted universal fact, viz. that a certain stable
configuration obtains; and (2) a type of force independently known to
exist. The argument seeks to prove that the stability in question is
impossible unless a second kind of force exists to balance the first
kind. In Kant's particular version of the balancing argument,
the universal fact is the constancy of mass density in a control
volume; and the given force is "original repulsion," whose
existence he proves in Proposition 4.) Proposition 7 then specifies
how attractive forces are to be understood, namely as the immediate
action of matter on other matter through empty space (and therefore at
a distance). Kant thus directly confronts the metaphysical question of
how to understand attraction that Newton attempted to avoid by
positing it merely mathematically. As Kant interprets the situation,
Newton "abstracts from all hypotheses purporting to answer the
question as to the cause of the universal attraction of matter
... [since] this question is physical or metaphysical, but not
mathematical" (4:515). In response to the "most common
objection to immediate action at a distance," namely "that
a matter cannot act immediately *where it is not*"
(4:513), Kant argues that action at a distance is no more problematic
than action by contact (whether it be by collision or pressure), since
in both cases a body is simply acting outside itself. Proposition 8
concludes by arguing that attractive forces act immediately to
infinity and by adding a "preliminary suggestion" (4:518)
as to how one might be able to construct the concept of cohesion
(which Kant understands as attraction that is restricted to
contact).



In the General Remark to Dynamics Kant addresses two main issues.
First, Kant considers how it is that the specific varieties of matter
(e.g., water as different from mercury) might be reduced, at least in
principle, to the fundamental forces of attraction and repulsion. The
second issue concerns the fundamental distinction between the
"mathematical-mechanical" and the
"metaphysical-dynamical mode of explanation". The former
mode of explanation, which is associated with the postulation of atoms
and the void, employs nothing more than the shapes and motions of
fundamental particles and empty interstices interspersed among them.
It contrasts with the metaphysical-dynamical mode, which employs
fundamental moving forces (e.g., attraction and repulsion) in its
explanations. Kant grants that the mathematical-mechanical mode has an
advantage over the metaphysical-dynamical mode, since its fundamental
posits can be represented (indeed, "verified" (4:525))
mathematically, whereas he repeatedly admits that the possibility of
fundamental forces can never be comprehended, i.e., their possibility
can never be rendered certain. However, Kant thinks that this
advantage is outweighed by two disadvantages. First, by presupposing
absolute impenetrability, the mathematical-mechanical mode of
explanation accepts an "empty concept" at its foundation.
Second, by giving up all forces that would be inherent in matter, such
a mode of explanation provides the imagination with more freedom
"than is truly consistent with the caution of philosophy"
(4:525).



Given that the bulk of Kant's matter theory is presented in the
Dynamics, it is unsurprising that it has received the greatest amount
of attention in the literature. Of particular note are discussions by
Buchdahl (1968, 1969), Brittan (1978), Kitcher (1983), Butts (1986),
Carrier (1990), Friedman (1990), Malzkorn (1998), Warren (2001, 2010),
Pollok (2002), Holden (2004), and Engelhard (2005). Friedman (2013) is
an extensive commentary, under the overarching theme that Kant in the
Dynamics aims to explain how certain features of body - density,
volume, static weight - become mathematizable, as Newton's
dynamics illustrates paradigmatically. Two chapters (by McNulty and
Warren, respectively) in McNulty 2022 add to our understanding of this
aspect in Kant's thought.


### 2.4 Mechanics



The third chapter of Kant's *Metaphysical Foundations*,
the Mechanics, concerns how it is possible to experience matter as
having a moving force, that is, how one matter communicates its motion
to another by means of its moving force. Kant begins, in Proposition
1, by clarifying how the quantity of matter is to be estimated before
stating, in Propositions 2-4, three Laws of Mechanics.



After first defining the quantity of matter and the quantity of motion
(or, in contemporary terms, impulse, *i*=*mv*), Kant
asserts that the quantity of matter, in comparison with *every*
other matter, can be estimated only by the quantity of motion at a
given speed (4:537). Kant's proof proceeds by way of
elimination. The quantity of matter, which is the aggregate of the
movable in a determinate space, cannot be estimated by counting the
number of parts it has, since, as was established in the Dynamics,
every matter is infinitely divisible. Nor can one estimate the
quantity of matter merely by considering its volume, since different
matters can have different specific densities. As a result, the only
universally applicable way of estimating the quantity of matter is to
hold the velocity of matter constant.



In Proposition 2, Kant states his First Law of Mechanics: the total
quantity of matter remains the same throughout all changes in matter
(4:541). His proof seems to rely (i) on the principle of the First
Analogy of Experience that no substance arises or perishes throughout
any change in nature and (ii) on the identification of what in matter
must be substantial. On this latter point, Kant quickly assumes that
the ultimate subject of all accidents inhering in matter must be the
movable in space, and that its quantity is the aggregate of the
movable in space. In his remark to this proposition, Kant explicitly
notes that there is a crucial difference between spatial and
non-spatial substances, since the latter, unlike the former, could
gradually fade away by degrees. (Kant cites the possibility of
consciousness as a concrete example.) Kant uses this difference to
argue that since the quantity of matter consists in a plurality of
real things external to each other that cannot fade away (as
consciousness might), the only way to decrease its quantity is by
division.



Kant's Second Law of Mechanics, stated in Proposition 3, is that
every change in matter has an external cause. (Immediately after this
principle, Kant adds in parentheses a version of the law of inertia
that is much closer to Newton's: "every body persists in
its state of rest or motion, in the same direction, and with the same
speed, if it is not compelled by an external cause to leave this
state" (4:543). Since Kant's Second Law of Mechanics is
not identical to Newton's law of inertia, it would require
argument to show that, and by means of what additional assumptions,
the former entails the latter.) The proof of the main principle
depends on the Second Analogy of Experience (which asserts that all
changes occur in accordance with the law of cause and effect and thus
entails that every change in matter has a cause) as well as on the
further assumption that matter has no internal grounds of
determinations (such as thinking and desiring), but rather only
external relations in space. In his remark to this proposition, which
clarifies this "law of inertia," Kant explains that
inertia is to be contrasted with life or the ability of a substance to
determine itself to act from an *internal principle*. Thus, a
body's inertia "does not mean a *positive striving*
to conserve its state" (4:544), but rather what it does
*not* do, its lifelessness.



Kant also asserts that the very possibility of natural science proper
depends on the law of inertia, since the rejection of it would be
hylozoism, "the death of all natural philosophy" (4:544).
In a later remark in the Mechanics, Kant explicitly objects that
"the terminology of inertial force (*vis inertiae*) must
be entirely banished from natural science, not only because it carries
with it a contradiction in terms, nor even because the law of inertia
(lifelessness) might thereby be easily confused with the law of
reaction in every communicated motion, but primarily because the
mistaken idea of those who are not properly acquainted with the
mechanical laws is thereby maintained and even strengthened"
(4:550). Kant goes on to point out that if inertia were to entail an
active force of resistance, then it would be possible that when one
moving body hits another, the moving body has to apply part of its
motion solely to overcome the inertia of the one at rest and might not
have any motion left over, as it were, to set the body at rest into
motion, which is contrary to experience (and Proposition 2).



Kant's Third Law of Mechanics, expressed in Proposition 4,
asserts the equality of action and reaction in the communication of
motion. Kant formulates a version of the Third Analogy of Experience
(according to which all external action in the world is interaction)
and suggests that the main point at issue in mechanics is establishing
that mutual action is necessarily *reaction*. Kant's
argument for this law is based on the following line of thought: (i)
if all changes of matter are changes of motion; (ii) if all changes of
motion are reciprocal and equal (since one body cannot move closer
to/farther away from another body without the second body moving
closer to/farther away from the first body and by exactly the same
amount); and (iii) if every change of matter has an external cause (a
proposition that was established as the Second Law of Mechanics), then
the cause of the change of motion of the one body entails an equal and
opposite cause of a change of motion of the other body or, in short,
action must be equal to reaction.



Kant's Laws of Mechanics have been discussed widely in the
secondary literature. Especially helpful discussions are Friedman
(1989, 1992), Brittan (1995), Westphal (1995), Carrier (2001), and
Watkins (1997 and 1998b). The most sustained and through discussion of
them is in Watkins (2019). Much of the modern tradition of scholarship
used to regard Kant's laws of mechanics as derived from, or even
identical with, Newton's three laws in the *Principia*.
Watkins (2019) shows that Kant's formulation and justification
of his laws was strongly influenced by a philosophy of nature stemming
from Leibniz, not Newton. Stan (2013) further corroborates these
findings. As a result, it is safe to say that Kant's foundations
of mechanics were significantly shaped by post-Leibnizian rationalism
too, not just Newton's mechanics. This fact is now reflected in
Friedman (2013), the most recent and detailed account of Kant's
interpretation of the conceptual foundations of mechanics. Stan (2014)
examines the conceptual link between Kant's theory of matter and
his laws of mechanics.


### 2.5 Phenomenology



The final chapter of the *Metaphysical Foundations*, the
Phenomenology, focuses on how the motion of matter can be experienced
modally. Specifically, Kant spells out conditions for us to judge
legitimately that a body possibly moves; is really in motion; or that
its motion is necessary. Its three Propositions specify (in
accordance, Kant suggests, with the results of the three previous
chapters) that (i) rectilinear motion is a merely possible predicate
of matter, (ii) circular motion is an actual predicate of matter, and
(iii) the equal and opposite motion of one matter with respect to
another is a necessary motion of that matter. In the General Remark to
the Phenomenology, Kant discusses the status of absolute space, which
had been presupposed by the possible, actual, and necessary motions of
matter at issue in the three main propositions, and explains that
since it is not itself an object of experience, it must be represented
by means of an idea of reason (in Kant's technical sense of
"idea," namely, as a concept for which a corresponding
object could never be given to us in intuition). Though we can never
know absolute space, it nonetheless functions as a regulative
principle that guides us in our scientific practice by forcing us to
look for further conditions for the conditioned objects we meet with
in experience. Kant's view that ideas of reason can function as
regulative principles is developed in the Appendix to the
Transcendental Dialectic in the *Critique of Pure Reason*.



Friedman's interpretation of the Phenomenology deserves special
mention. According to him, in light of Kant's rejection of
Newton's absolute space and time, he must give an account of the
concept of true motion - as change of true place over time
- presupposed by classical mechanics. To this end, Kant
"views the laws of motion as definitive or constitutive of the
spatio-temporal framework of Newtonian theory," hence these laws
"count as a priori" for him (1992, 143). The reason Kant
takes them as constitutive is the following. In Kant's
Phenomenology, the three Newtonian laws *define* a concept of
true motion: the true motions of bodies *just are* those that
obey the dynamical laws. Moreover, the concept must be given
"objective meaning in experience," viz. measured
empirically. To do so, Kant views Newton's three laws as holding
primarily in a privileged system of reference, namely the
center-of-mass (CM) frame of the system of the world, which the
CM-frame of our Solar System approximates to a very good degree. Thus,
by measuring the motions of bodies relative to this frame, we produce
objective experience of these motions. However, this frame must first
be *located*. To do so, Kant thinks, we must likewise count the
law of universal gravitation as a priori, not empirical-inductive. If
we know a priori that all bodies in the Solar System necessarily
attract each other, from their observable, mutually-induced
accelerations we can infer their masses. In turn, knowing their masses
will enable us to locate the CM-frame of the system. (In a system of
bodies, the center of mass is the point relative to which the
bodies' distances are in inverse proportion to their masses.)
With respect to this distinguished frame, the motions of bodies count
as their true motions, Friedman claims. However, the CM-frame of the
Solar System is just approximately an inertial frame. Scientific
inquiry must ultimately look beyond it, to better and better
approximations of an inertial frame. And, Kant's absolute space
is just the concept that directs our search for such approximations.
More recently, Friedman updated and expanded his interpretation in
(2013). In view of recent scholarship, Friedman has now made a
convincing case that it is *Kant's* laws of mechanics
- not Newton's three laws, as claimed in (1992) -
that define the privileged frame (Kantian absolute space) relative to
which bodies have true motions, in the Phenomenology.



Prior to Friedman's interpretation, there is relatively little
secondary literature that discusses Kant's Phenomenology.
Carrier (1992) gave an alternative to Friedman's account of Kant
on absolute space. Two chapters (by De Bianchi and Massimi,
respectively) in McNulty 2022 add to our understanding of this aspect
in Kant's thought. Stan (forthcoming) assesses the Phenomenology
in its early-modern context.


## 3. Laws



The Scientific Revolution ushered in a new type of knowledge, based in
laws of nature. Kant's natural philosophy thus gave that notion
a central place. In particular, he analyzed it, he used it to single
out certain laws as fundamental, and he weaved it into his picture of
systematic science. We go over these aspects in turn.



**Nomic import**. For the Critical Kant, a law is a
judgment, or proposition, that is *necessary* and
*imposed* by a faculty of the mind--specifically, the
understanding or reason. In the realm of theoretical knowledge, laws
determine the object they govern. (In contrast, laws of practical
reason bind their subjects, by mandating an action or a maxim for
behavior.) There are several classes of Kantian laws: transcendental
(e.g., the pure principles of the understanding); metaphysical (such
as his three laws of mechanics); and empirical (in particular, the
laws of chemistry).



The above picture, which Watkins 2019 articulates and defends in
detail, gives center stage to the thought that laws are partially
grounded in the natures of knowers like us--specifically, in the
faculties (and their a priori structures) that Kant investigated in
the First Critique. Other construals leave this key element out,
and rely on other resources from his theoretical philosophy. Some have
proposed a 'necessitarian' account, in which Kantian laws
are grounded in the natures of the things they are about, thus
natures--the bodies' essential properties and
powers--are their necessity-makers (Messina 2017, Massimi 2017,
Engelhard 2018). Others think that certain propositions, or judgments,
count as laws in virtue of their place in a best-system ordering of
our knowledge as directed by Kant's ideas of reason (see, e.g.,
Kitcher 1986 or Breitenbach 2018). These two interpretive approaches
turn Kant into a precursor of two major contemporary notions of law,
by Fred Dretske and David Lewis, respectively. Watkins argues that, if
read carefully, Kant in fact offers a view that *combines* the
strengths of the two pictures above: it retains the necessitarian
dimension while emphasizing the role of our acts--of us qua
spontaneous epistemic subjects--in regard to nature as the object
of unified experience (Watkins 2019, esp. Chapter 2).



In philosophy of physics, Kant privileged three laws: Conservation of
Mass, the Law of Inertia, and the Equality of Action-Reaction. He
regarded them as fundamental for mechanics as he understood it, viz.
qua theory of momentum exchanges, or 'communication of
motion.' And, these laws lie closest--explanatorily and
evidentially--to the Analogies of Experience, the most general
principles of theoretical knowledge, in his system. Specifically, the
Second Analogy is a key premise in his derivation of his 'second
mechanical law.'



Though somewhat intermittently, Kant reflected as well on empirical
laws, viz. nomic judgments that govern some restricted class of
objects or properties given a posteriori. He thought that, to count as
a rational science, a body of knowledge must include empirical laws
systematically connected. In the last decade, this long-neglected
aspect of his doctrine has began to receive the attention it deserves.
Scholarship has tended to concentrate on two main aspects. One is
Kant's basis for counting certain propositions as empirical
laws. Some see them as earning that status in virtue of their place in a
best-system organization of our knowledge; others think they become
laws at a certain stage in the process of inquiry (provided the
inquiry is guided by specific normative Kantian principles, e.g. his
Postulates of Empirical Thinking). The other aspect is whether
empirical laws count as knowable by Kant's lights. Here too, the
field appears divided in two camps. Some think that, pace his positive
account of empirical laws, knowledge of them is beyond our reach (see,
for instance, Kreines 2017). Others have argued that, under the right
conditions, research can lead to the discovery of empirical laws in
that particular domain, e.g. chemical phenomena (see, for instance,
McNulty 2015 and Cooper 2023).



**Hypotheses**. Natural science after 1650 often relied
on 'hypotheses,' or knowledge claims for which evidence
was less than conclusive. In optics, electricity, magnetism, heat
science, chemistry, and natural history hypotheses were rampant.
Accordingly, some early-modern figures (e.g. Christian Wolff and
Emilie du Chatelet) developed doctrines to address this fact
about science then. Kant did too. In particular, he sought to identify
normative criteria for acceptable hypotheses, viz. thresholds that
candidate hypotheses must meet if theorizing from them is to count as
legitimate. He defended for three such criteria in his
'Discipline of Pure Reason in regard to Hypotheses,' a
part of the First Critique. The first is that a candidate hypothesis
has to be about things that are 'really possible' in
Kant's sense; namely, *spatio-temporal* objects and
properties. (This rules out, for one, hypotheses about God being the
'cause of gravity,' which some Newtonians had peddled in
the early 1700s). The second is that a hypothesis must be about
objects and processes *causally* connected--more broadly,
connected via the categories of relation--to "that which is
actually given, and so is certain" (B798). Helpful studies of
Kant's account of hypotheses in their scientific and
philosophical context are Leduc 2013, Vanzo 2013, McNulty 2015, Cooper
2023, and Demarest & van den Berg 2022.


## 4. Physics: The Post-Critical Period (*Opus postumum*)



Kant's interest in physics continued after the publication of
the *Metaphysical Foundations*, in fact, until the very end of
his productive years. Though Kant never completed a manuscript that
could be put forward as a publication, the various notes, sketches,
and drafts on topics in physics that he was working on intensively
during this time (especially after 1796) were gathered together over a
century after his death and published as his so-called *Opus
postumum*. It is a highly fragmentary work where Kant's
thoughts remained in flux through the final parts, written ca. 1802.
Accordingly, scholarship has sought to recover the evolution,
background and import of his reflections, rather than some stable
doctrine, which it mostly lacks. Below we touch on his reason for
writing the *Opus* at all; two major themes relevant to
philosophy of science; and some implications for his natural
philosophy.



Why did Kant around 1796 begin to draft a new work? Most interpreters
agree that he thought the *Opus* aimed to fill a
'gap' of sorts. In letters from 1798, he speaks
cryptically about there being a 'gap' in his Critical
philosophy. Many have concluded the *Opus* aimed to fill
*this* gap, and so they turned their attention to identifying
it. Some claimed it was a gap in Kant's theory of matter above
(in particular, his accounts of mass and density); others saw it
between his constitutive and regulative approaches to metaphysical
foundations for science; yet others have said it was a lacuna in his
schematism of the categories. Lately, Howard (2023) has argued that it
was a missing part in the 'system of philosophy' as a
whole (*hiatus in systemato*, 21:476). He reads that as the
need to mediate--by devising a bridge doctrine--between
Kant's metaphysical foundations of natural science and
'physics as a system.'



This reading elucidates two obscure parts in the Opus. One is
Kant's repeated attempts to articulate a
'transition.' In the picture that Howard sketches, the
transition would be the very mediating doctrine above: between a
theory in metaphysics and a body of empirical knowledge. The other is
Kant's repeated attempts there to *define* physics such
that he can defensibly say it counts as systematic science. For
instance: "Physics is systematic investigation of nature through
empirically given forces of matter insofar as they are connected to
each other in a system" (22:298). Note just how broad his
definition is--it makes the various 'forces'
theorized by early-modern chemistry, heat science, botany and animal
physiology count as part of physics qua organized doctrine.



A central theme in the *Opus* is Kant's attempt to give
an a priori proof that a certain entity must exist. He calls it the
'ether,' and also 'caloric.' He thought of it
as a sort of universal substrate--a fluid-like medium that fills
all of physical space, supports 'moving forces,' and
carries their actions from one place to another (much like pressure
applied at a point in an incompressible fluid travels across it to
other locations). Kant saw it as his task to give a
'deduction' of this medium, viz. a species of
non-empirical evidence that it exists. Generally interpreters take his
*Atherbeweis* to be an actual deductive argument, which
they have tried to reconstruct (see, e.g., Forster 2000 and Hall
2006).



Another major theme there is Kant's quest for an
'elementary system of the moving forces' of nature. He
means a methodic, or well-ordered, derivation of the
'specific' properties of matter. They are specific in that
none counts as general, or common to all matter; but none is too
particular or transient either. Rather, each is a feature of some
broad class of material behavior. Among the properties Kant ponders at
this juncture are 'heat,' 'weight,'
'elasticity,' 'solidity,' and the like. He
takes these properties to be associated with 'moving
forces': they result from some force being exerted (e.g.,
gravity induces weight) or they cause some force of motion (e.g.
elastic rebound pushes back the body that compressed an elastic
material). So, an elementary system of properties will be ipso facto a
system of forces as well. In regard to it, Kant sought a *rational
heuristic*: a recipe or criterion for a taxonomic account (of
these broadly-specific properties) that is a priori, viz. inferable
before they are empirically discovered.



Lastly, what did Kant think his ether--and, alternatively, his
'elementary' system--would achieve? Here too,
scholarship is somewhat divided. Older accounts thought the
'ether, or caloric' was Kant's basis for the bridge
doctrine, or transition, between metaphysics and empirical physics.
More recently, Howard (2023) has argued for a different line. Namely,
that Kant saw the ether deduction (and the taxonomy of
'elementary' forces) as the basis for an account of what
makes empirical physics a systematic body of knowledge--or would
make it so in the long run, if properly pursued.



The *Opus postumum* has long been a topic of interest
especially insofar as it offers the hope of clarification and
development of central issues in Kant's Critical philosophy.
Much scholarship on it was in German (Adickes 1920, Hoppe 1969,
Tuschling 1971, Blasche 1991, and Emundts 2004). Then it received
increased attention in English, with discussions by Friedman (1992,
chapter 5), Forster (2000), Edwards (2000), Guyer (2001), and
Hall (2006, 2009). Recent views on the *Opus* and its reception
are presented in Basile (2013) and Basile & Lissy (2022). Howard
(2023) is an overview of its key issues, and a novel reading of the
natural-philosophical issues Kant tackled there.


## 5. Biology



Kant was interested not only in the inanimate bodies studied by
physics, but also in living organisms, which he viewed as deserving of
additional philosophical scrutiny due to their special status, for two
reasons. First, the symmetries and regularities of their structure and
workings make them "in the highest degree contingent" (5:
360) and "mechanically inexplicable." Second, organisms
come from their own kind; grow their own functional parts, or organs,
and repair them if they become damaged; and do in general exhibit
homeostasis. These facts about them prompt Kant to claim that an
organism is "cause and effect of itself" (5: 370). In a
sense, a living entity is self-caused, and indeed by a causality that
we can grasp solely "under the idea of purposes as a
principle" (5: 383). Organisms, in other words, are
"natural purposes," or naturally occurring ensembles of
parts purposefully fitted and working in unity.



It is just these peculiarities that create philosophical challenges
not met with in physics and chemistry. Kant tackles them in the latter
half of his third magnum opus, the *Critique of the Power of
Judgment* (1790). In particular, he claims that attempting to
explain organisms creates an inevitable conflict within reflective
judgment, the faculty we employ in empirical concept formation,
taxonomic classification, and the explanatory unification of
particular laws. Kant describes that conflict in the Antinomy of
Teleological Judgment (*CJ* SSSS 69-78; 5:
385-415). The Antinomy obtains between two regulative
principles, herein prefixed by an 'R' to distinguish them
from two similar *constitutive* principles that Kant briefly
discusses in the same place:



> 
> 
> **R-thesis**:
>   
> 
> All generation of material things and their forms must be judged as
> possible in accordance with merely mechanical laws.
> 
> 
> 
> **R-antithesis**:
>   
> 
> Some products of material nature cannot be judged as possible
> according to merely mechanical laws (judging them requires an entirely
> different law of causality, namely that of final causality) (5:
> 387).
> 
> 
> 



A great deal of scholarship has aimed to elucidate this antinomy, and
in the last decade these interpretive efforts have gained new momentum
(see also the entry "Kant's Aesthetics and
Teleology" in this encyclopedia). In particular, sustained
attention has been given to three questions: In what sense are
organisms inexplicable mechanically, for Kant? What exactly is the
contradiction that generates the Antinomy? And precisely how does Kant
solve it?



First, clarification is needed about Kant's claim that organisms
cannot be explained 'mechanically.' In the Enlightenment,
'mechanical' was used in at least two senses: broadly, to
denote explanation by physico-chemical laws of *efficient*
causation; and narrowly, restricted to the theory of 'compound
machines,' or assemblies of movable parts used to displace loads
or transfer momentum, e.g. engines and watches. McLaughlin (1990)
takes Kant to mean 'mechanical' in this latter sense. In
artificial machines, parts *precede* the whole temporally, and
*determine* it: the machine's configuration results from
summation over the parts; and its laws of operation are the same as
those governing the parts. But organisms are different: the whole
precedes its parts, or organs; and it has functions and aim-directed
behaviors not present in them. Hence, organisms are mechanically
inexplicable. Ginsborg (2004) construes Kant's claim along
different lines, in two senses. According to one, to explain something
mechanically is to show that its production results, or would result,
from the "unaided powers of matter as such," i.e.
Newtonian forces of attraction and repulsion, and "crude
unorganized matter," or arbitrary initial conditions such as
masses, charges, configurations and motions. Organisms are
inexplicable in this sense because their arising would be "in
the highest degree contingent." This is the notion operative in
the Antinomy of Teleological Judgment, Ginsborg contends. But, she has
argued, there is a second, stronger sense. Not only are organisms
inexplicable by reduction to the structure and powers of their
organs--rather, their symmetries and regularities and powers are
not explicable from the "moving forces" of their
*inorganic* particles either. Organism-level powers are not
vector sums or integrals of physico-chemical forces that accelerate or
reconfigure sub-visible particles.



Second, interpreters have also scrutinized the exact nature of the
Antinomy. Insofar as the thesis and the antithesis are regulative
principles, they may be seen as research policies, and those can be
pursued separately, at different times, hence with *no*
contradiction. Then why are they antinomic? Watkins (2009, SS 2.2)
explains that the two theses do not just recommend explanatory
policies but express commitments to judging in a determinate way about
biological phenomena. As such, they would be contradictory
commitments: to judge organisms purely mechanistically *and* to
judge them non-mechanistically. Teufel (2011) reads the two theses as
hypothetical imperatives, imposed upon reflective judgment by
practical reason. The Antinomy ensues, he contends, because they
demand contradictory courses of cognitive conduct: to judge that
mechanistic laws suffice to explain organisms *and* that they
are insufficient for that task.



Moreover, for each antinomy in the *Critique of Pure Reason*,
Kant derives both thesis and antithesis by deductive argument from
alleged a priori premises. But, in the Antinomy of Teleological
Judgment, he does not take that route. This makes quite pressing the
question of his justification for the two R-claims above. As to the
R-thesis, one might think that it can be easily deduced from
Kant's three Laws of Mechanics. However, we must remember that
those laws are constitutive principles, whereas the R-thesis is
regulative. A better interpretive avenue is that Kant's three
laws constrain the spectrum of particular empirical laws, including
laws for biology, that scientists may seek. The constraint is that the
Laws of Mechanics are basic: no other laws of material nature explain
or ground them, while they must explain *all* other laws,
directly or indirectly. So, for biological regularities to be turned
into genuine laws, they must be integrable with the system of already
known mechanical laws, and ultimately explained by the three Laws of
Mechanics, the basis for all "interconnected experiential
cognition" (5: 386). Even less well understood is Kant's
justification for the R-antithesis. Not only is there no official
proof, it is not easy to know what the proof must establish. The
R-thesis strongly implies that at issue is the *origin* of
organisms; in contrast, the R-antithesis is about judging their
*possibility*, which has to do with structure and functioning
not production (Ginsborg 2001, 236). These are different aspects of
organisms, and proving that they are mechanically inexplicable would
require different arguments.



Third, Kant's solution to the Antinomy has received sustained
attention. In the first two Critiques, his general approach is to
argue, for each antinomy, that both thesis and antithesis presuppose
Transcendental Realism, insofar as they make claims about things in
themselves; and to adduce Transcendental Idealism as the vantage point
from which the initial contradiction is solved. However, that is not
Kant's approach to the Antinomy of Teleological Judgment.
Accordingly, interpreters have sought to reconstruct a different
solution on Kant's behalf. Quarfood (2004) and Forster
(2008) take the notion of an intuitive understanding (expounded at 5:
401-10) as the key to Kant's answer. In particular,
Forster explains, intellects like ours are discursive: they grasp
objects through concepts as 'parts' assembled in
temporally-extended judgments, and so mechanistically, he alleges.
Hence, we need the concept 'purpose' to judge organisms
solely because we have discursive intellects. However, an intuitive
understanding could grasp organisms without purpose-concepts and
regulative principles about judging teleologically. Hence, for this
understanding, the R-thesis and R-antithesis need not be true, and so
no antinomy arises. Breitenbach (2008) contends that the Antinomy is
solved, or rather dissolved, if we note that thesis and antithesis are
not really contradictory but complementary. They govern (reflecting)
acts of judging in different contexts: the mechanistic R-thesis
regulates explanatory conduct in empirical biology, whereas the
R-antithesis governs ordinary judging about living nature as
experienced in everyday, pre-theoretical encounters with it. The two
principles epitomize "two different views on nature," and
are compossible (466). They just appear to be in contradiction; in
fact, they complement each other, indispensably. McLaughlin (1990)
submits that the Antinomy does not arise *simpliciter* but only
under a special assumption, viz. that everything in nature is
objectively explainable by us (162). Reject the assumption, and the
contradiction vanishes, insofar as the R-thesis and the R-antithesis
can both be true, but for different domains: the former holds for
phenomena explainable by us, the latter for phenomena not so
explainable. McLaughlin does not offer a general criterion for
'explicability by us' attributable to Kant, however, apart
from mechanical explicability. Ginsborg (2008) focuses on how the
contradiction might be resolved in the context of scientific
investigation rather than in abstract, general philosophical terms.
Building on Kant's remark about subordinating mechanism to
teleology (5: 414), Ginsborg contends that natural historians and
physiologists may well explain the origin and workings of organisms
'mechanistically,' purely by physico-chemical laws,
*provided* they apply them not to "crude" but to
"organized" matter, i.e. organic stuff already endowed
with a "formative drive," a generic disposition to grow,
reproduce and self-maintain. In turn, this organized matter itself is
unintelligible without appeal to purpose and design terms. Thus,
biologists 'subordinate' mechanism to teleology, and so
the antinomic conflict between them is removed.



While these interpretive proposals collectively advance our
understanding of how the Antinomy might be defused, each has its share
of philosophical difficulties, as spelled out in Watkins (2009).
Moreover, few of these construals heed close to Kant's words,
which suggest strongly that his solution relies *indispensably*
on a "supersensible ground" (5: 413), or explanatory
principle located in the noumenal world. That ground *could*
unite both mechanism and teleology, by making both antinomic theses
jointly derivable or explainable from it. However, since humans can
have "no positive determinate" knowledge of any
supersensible entity, the Antinomy is solvable in principle rather
than solved in fact. Kant (at least in his official solution) does not
seem to provide the details for an actual account of how the conflict
at the heart of biological explanation is removed. The limitations
that beset our understanding of Kant's philosophy of biology so
far, some of which we have expounded above, continue to drive
interpretive efforts to cast more light on that part of his doctrine,
as exemplified in Goy and Watkins (forthcoming).



In addition to the works mentioned above, Kant's Antinomy of
Teleological Judgment has been usefully discussed in Allison (1991),
Cohen (2004), Ginsborg (2001), Kreines (2005), Steigerwald (2006),
Zanetti (1993) and Zumbach (1984); see also the entry
"Kant's Aesthetics and Teleology" in this
encyclopedia.


## 6. Chemistry



Kant draws a distinction between 'proper' and
'rational' science. The former denotes any discipline that
is strongly mathematized and relies on 'apodictic' laws,
supported by conclusive evidence; Newton's gravitation theory is
the epitome of that. The latter indicates a body of propositions that
is systematic, viz. explanatorily integrated, and includes laws. Kant
counted chemistry as a poster child for it. He lived through a
watershed change in that discipline--from Georg Stahl's
phlogiston theory to Lavoisier's renovation of chemistry, its
core processes, and taxonomy of stuffs.



In *Metaphysical Foundations*, Kant argued that, in a
'proper' science, its basic concepts correlate with
mathematical notions, and the warrant for that correlation comes from
metaphysics, e.g. via a picture of the essence of matter. Thereby,
metaphysics supplies principles for the construction' of the
domain-specific concepts of that science. But, he continued, no such
metaphysical warrant is available for correlating basic
*chemical* concepts with mathematics; when such correlations
occur, they are known a posteriori. (He was aware, of course, that
chemical practice in his time had discovered quantitative facts, e.g.
about the ratios of reagents in a specific reaction.) Thus, he
concluded, not only is chemistry not a proper science, but it is not
likely to become one ever (4:471).



Still, he counted it as a 'rational' science: he thought
chemistry owes its systematic order to ideas of reason. Specifically,
to certain domain-specific artificial concepts introduced for the sake
of descriptive and explanatory unification of many local regularities
established empirically. For instance, that all solid metals then
known 'calcinate,' i.e. form heavier compounds when heated
in air; and can be obtained from these compounds, under the right
laboratory conditions.



After 1790, Kant apparently moved to endorse Lavoisier's new
taxonomy of chemical substances, and also his picture of the
'caloric,' or 'ether'--a species of
'imponderable fluid' endowed with causal properties
relevant to, among others, heat exchanges, 'combustion'
(oxidoreduction) and phase transitions. And yet, as recent research
has established, even in the post-Critical years Kant held fast to his
above requirements for a science to count as proper. In light of that,
late-Enlightenment chemistry--Lavoisier's great
transformation and Kant's 'ether deduction'
notwithstanding--still counted for him as a rational, but
improper, science.



Interest in Kant's philosophy of chemistry has surged in the
last decade. McNulty (2014, 2015, 2016) is an especially helpful and
clear series of studies of Kant's engagement with that
rapidly-changing science. Blomme (2023) contributes to the question of
what chemistry was, for Kant. Older discussions of his views on the
topic are Carrier (1990, 2001) and Friedman (1992).


## 7. Psychology



Kant's views on psychology are intimately bound up with his more
general position in the philosophy of mind (see the separate entry on
this topic.) He distinguishes between rational and empirical
psychology and, in the *Critique of Pure Reason*'s
Paralogisms, denies that rational psychology contains arguments that
could justify any substantive principles (especially concerning our
immortality). Thus, only empirical psychology, it seems, could be
possible as a science. However, if Kant continues to maintain that
science requires a pure part, and denies that rational psychology
contains any substantive knowledge that might constitute the pure part
of psychology, then it follows that empirical psychology cannot
qualify as science proper either.



And yet, he did not count it as a proper science either. If we ask
why, we can discern two broad answers in the secondary literature.
Older interpretations thought it was because mathematization (a mark
of 'proper' science) is not feasible in psychology.
Supposedly, it is because psychic phenomena are in time--it is
their ordering structure, or a priori form--and time qua form
lacks enough features to support strong mathematization. (It is a
1-dimensional manifold representable as a line: too narrow to support
constructions in pure intuition, which mathematization would require.)
A more recent construal has it that empirical psychology lacks genuine
laws. Kant leaves open the prospect for it coming to have 'laws
of association,' viz. knowledge of patterns of constant
conjunction between some of our inner representations. Such patterns,
discoverable by introspection, may resemble laws properly so-called.
But, as some have argued recently, that is not enough. Genuine laws
require stronger warrant--primarily, evidence from successful
prediction confirmed by experiment or from precise observations. That
sort of evidence, interpreters believe, is not available in empirical
psychology. Which makes it an improper science by Kant's lights:
even more improper than chemistry, which arguably does avail itself of
genuine laws explicated as above. A scholar who endorses the first
reading above is Hatfield 1992; for the second, see especially McNulty
2014.



Unsurprisingly, Kant influenced empirical theory in early 19th-century
Germany. (However, it was his transcendental psychology in the
*Critique of Pure Reason*, not his views on empirical
psychology, that proved to be most influential.) In particular, his
doctrine of space and time as forms of sensibility was given a
naturalistic construal in the physiology and psychology of perception.
Since for Kant an a priori form of intuition need not be innate, it
could unfold in the course of sensory experience. This led J.G.
Steinbuch (1811) and C. Th. Tourtual (1827) to investigate empirically
how spatial representation arises--specifically, how particular
subjects become able to see things as arrayed in space, perceive
objects as three-dimensional, and represent spatial structures more
generally.



Hatfield (1990, Chapter 4) surveys these post-Kantian developments in
more detail. Kitcher (1990) presents a detailed argument for the role
of transcendental psychology within Kant's *Critique of Pure
Reason*. Sturm (2001) argues that Kant's critical comments
about psychology are primarily directed against introspection-based
conceptions of psychology.


## 8. Other Sciences: History, Physical Geography, and Anthropology



Though Kant is sometimes quite strict about what qualifies as science
"proper," we have seen that he clearly accepts that other
disciplines, such as chemistry and psychology, can be scientific in
some other sense. (Kant is also extremely interested in providing an
explanation of the nature and origin of organisms, which is central to
an account of biology. For a more detailed description of Kant's
stance on biology, see the entry on his views on aesthetics and
teleology.) However, his explicit reflections about science (taken
broadly so as to encompass not only
"*Wissenschaft*" but any kind of
"*Lehre*" or "*Kunde*") extend
even further so as to include bodies of cognition such as history,
physical geography, and anthropology. (Even in the *Metaphysical
Foundations* Kant leaves room for such "scientific
cognition" by dividing the doctrine of nature into natural
science -- e.g., physics -- and the historical doctrine of
nature, which is separated further into natural description and
natural history, 4:468). In the case of historical sciences, Kant
views its cognitions as related not by subordination (as is the case
in physics), but rather by coordination, since historical facts cannot
be derived from each other, but rather merely related to each other in
space (geographically) and time (chronologically). Kant views
anthropology as overlapping to some extent in subject matter with
empirical psychology. What enables the coordination of facts in each
of these domains is not a theoretical, but rather a practical idea. In
the case of history, it is the idea of reason (or freedom) that
provides a guiding principle of coordination (cf. Kant's
"Idea for a Universal History from a Cosmopolitan Point of
View"), while anthropology is framed by the cosmopolitan moral
ideal of the world-best. Thus, it is clear that Kant's emphasis
on physics throughout his career did not blind him to the value of
other sciences, nor did it keep him from reflecting creatively on how
best to account for them from the perspective of his Critical
philosophy.



Kant's views on history are discussed by Yovel (1980) and, more
recently, Kleingeld (1995 and 1999). Kant's *Physical
Geography* has been discussed by Adickes (1911). Brandt (1999),
Wood (1999), and Makkreel (2001) have recently published important
discussions of Kant's anthropology. Sturm (2009) is a
comprehensive monograph on Kant's philosophy of psychology and
anthropology.