## 1. Theoretical reason: reason's cognitive role and limitations



In the first half of the *Critique of Pure Reason*, Kant argues
that we obtain substantive knowledge of the world through two
capacities: sensibility and understanding. Empirical judgments depend
on *both* sensory experience *and* concept formation.
Kant emphasizes the solidity of empirical knowledge gained this
way.



In the next large section, Kant discusses "theoretical
reason"--especially philosophical reasoning. The
"Transcendental Dialectic" attacks philosophical efforts
to gain knowledge of a "transcendent" world, that is, a
world beyond that revealed by the senses. "Dialectic,"
says Kant, is "a *logic of illusion*" (A293/B349).
Here, reason appears mainly as a source of empty or false ideas.



Many readers fall away before the final section of the book, where
Kant reviews his overall method--his own use of "pure
reason." The closing "Transcendental Doctrine of
Method" considers reason in terms of its
"discipline," "canon,"
"architectonic," and "history."



This makes it easy to interpret the *Critique of Pure Reason*
as simply critical of reason--as dismissing its claims to give us
knowledge. But if this were all that Kant intended, how should we
understand his own philosophical reasoning? Kant certainly wants to
show the limits of reason. But he also needs to show how reason can
play a constructive role. He does this in at least three ways: he
relates reason to empirical truth (SS1.1
 below); he explores reason's role in scientific enquiry
 (SS1.2
 below); and he explains the benefits of appreciating reason's
limits (SS1.3
 below).[2]



In addition, when we engage in philosophical reasoning, we ought to
understand the capacity that we are using. As Kant puts it, reason
must take on "the most difficult of all its tasks, namely, that
of self-knowledge" (Axi). The first *Critique* begins
this task, but does not complete it (SS1.4
 below).


### 1.1 Reason and empirical truth



Reason has an important role in our pursuit of knowledge. Kant writes:
"the law of reason to seek unity is necessary, since without it
we would have no reason, and without that, no coherent use of the
understanding, and, lacking that, no sufficient mark of empirical
truth..."
 (A651/B679).[3]
 Kant says relatively little to develop this claim, and the issue has
not attracted much commentary. (But cf. Walker 1989: Ch. 4; Guyer and
Walker 1990;
 Kant's theory of judgment,
 SS1.3.)[4]



However, the basic idea is clear from his text. We form judgments
about the world around us all the time. We see a hand in front of us
and judge it to exist; after a dream, we judge ourselves to have been
dreaming and the dream's contents to be illusory; we see the sun
rise and assume that it orbits the earth. (Obviously this last
judgment is false: see
 SS1.2
 below.) Kant devotes much effort to show how all these judgments rely
on *categories*. Fundamental concepts such as cause and effect
structure all our judgments. A belief that conforms to these
categories meets the "formal" conditions of truth.
However, unless we are fundamentally confused about something, all our
beliefs meet these
 conditions.[5]
 So how do we decide which of our beliefs are true, and which
mistaken?



Kant begins by noting that only *judgments* can be mistaken:
"It is correctly said that the senses do not err; yet not
because they always judge correctly, but because they do not judge at
all" (A293). For example, there is no error involved in
dreaming, however incoherent or fantastical the dream may be. But if
someone got confused and supposed that a dreamed-of event had really
happened, then she would be making a judgment--a false one. So
Kant claims, "error is only effected through the unnoticed
influence of sensibility on understanding, through which it happens
that the subjective grounds of the judgment join with the objective
ones" (A294). In the example, someone confuses a subjective
ground of judgment ("*I* had this dream") with an
objective one ("*these events* took place"). As
Kant puts it in the *Prolegomena*:



> 
> The difference between truth and dream... is not decided through
> the quality of the representations that are referred to objects, for
> they are the same in both, but through their connection according to
> rules that determine the combination of representations in the concept
> of an object, and how far they can or cannot stand together in one
> experience. (4:290)
> 



How does reasoning help here? In the famous "Refutation of
Idealism" (added to the second edition of the *Critique of
Pure Reason*), Kant writes: "Whether this or that putative
experience is not mere imagination [or dream or delusion, etc.] must
be ascertained according to its particular determinations and through
its coherence with the criteria of all actual experience"
(B279).



To see what Kant means, consider a simple example. Suppose that our
dreamer believes they have won a lottery, but then starts to doubt
this belief. To decide whether it is true, they must ask how far such
a belief connects up with their other judgments, and those of other
 people.[6]
 If it fails to connect up (they check the winning numbers, perhaps,
and find no match with the actual ticket), the dreamer must conclude
that the belief was false.



We can also see the logic here in terms of Kant's claim that
reason is "the faculty of the unity of the rules of
understanding under principles" (A302/B359). One fundamental
principle of experience is that we inhabit a single world in space and
time. Therefore, all true judgments must find a place within a single,
unified experience of the world. Reason seeks consistency. It helps
separate factual mistakes from genuine knowledge by referring to this
unifying condition.


### 1.2 Reason in science



The principle of reasoned unity also applies to scientific judgments
and theories. However, it takes a more complex form, since science
seeks universal laws.



Kant claims that reason is "the origin of certain concepts and
principles" (A299/B355) independent from those of sensibility
and understanding. Kant refers to these as "transcendental
ideas" (A311/B368) or "ideas of [pure] reason"
(A669/B697). He also defines reason as a "faculty of
principles" (A299/B356). But what are these concepts and
principles, and can they be justified? Since Kant also argues that
"ideas of reason" often lead to error and contradiction,
these are serious questions.



One sort of error arises when we claim knowledge of objects beyond
sensory experience, such as God or the soul. Another sort arises when
we form "transcendental ideas." We may try to
conceptualize the ultimate basis of everything that exists, such as
the universe as a whole. Kant refers to these as "world
wholes" or cosmological ideas. As discussed in a moment
 (SS1.3),
 when we claim objective knowledge about these, we fall into
contradictions or "antinomies." For example, Kant holds
that we can argue equally well for the opposing claims: that the
universe has a beginning in time *and* that it does not.



Scientific enquiry assumes that the world forms a well-ordered,
systematic unity where all events can be subsumed under causal laws.
This is not just the idea that events have causes--an inescapable
assumption that Kant refers to as a category of the understanding.
Rather, the claim is that universal laws underlie all particular
relations of cause and effect. This is what Kant has in mind when he
speaks of the "unity of reason" in the first
*Critique*. (See e.g. A302/B359, A665/B693, A680/B780.)



As an illustration, consider Copernicus's hypothesis that the
earth orbits the sun. This hypothesis contradicts our everyday
perceptions--perceptions that we normally rely on without
question. Historically, Galileo's observations were a turning
point. The telescope gave a much more extensive picture of planets and
stars, casting doubt on our everyday perspective. Although a new
scientific tool is involved (the telescope), the reasoning is quite
ordinary. In the face of inconsistent appearances, we must decide
which perspective to endorse--just as we may judge that an
impression is merely an optical illusion, or a dream is only a
 dream.[7]



For Kant's account of scientific reasoning, Newton is more
important (Bxxii n; cf.
 SS1.4
 below). Newton's laws of gravity unify Copernicus's
hypothesis and Galileo's observations--and much more
besides (A663/B691). The laws are universal: they extend to all
heavenly objects, not only the sun's motion relative to the
earth.



But we can never experience all events. However extensive our
experiences may be, and however many people's experiences we
draw on, experience is finite. As such, it can never justify the
principle that laws are genuinely universal, or that these laws will
continue to hold in the
 future.[8]
 Kant argues that reason is justified in adopting these principles
(and others). However, he makes a subtle distinction. These principles
should guide scientific inquiry--in Kant's term, they are
"regulative." But they do not provide *knowledge*
of the world--as Kant puts it, they are not
"constitutive." (See e.g., Buchdahl 1992; Friedman 1992c;
 Kant's theory of judgment,
 SS4.2.)



For Kant, the categories of the understanding are the paradigm example
of constitutive ideas. For example, the category of causation
structures all our knowledge. We do not perceive cause and effect
directly. Instead, experience is only possible because we
*assume* events have causes. (See
 Kant and Hume on causality
 for more detail.) Although we can make mistakes about specific
causes, we also have the following knowledge about cause and effect:
we *know* that every event has a cause. The category of cause
and effect *constitutes* experience--it enables us to
experience the world.



By contrast, regulative principles govern our investigations, without
offering any guarantee as to what we will find. Science seeks the
greatest possible completeness and systematicity (cf. Guyer 1989 &
2006, Abela 2006, Mudd 2017). Although scientists often pursue highly
specialized inquiries, science also has a *regulative* goal. It
must integrate all findings under the most all-encompassing laws.
Newton's laws have been modified by general relativity, for
example, pointing to the open-ended, "regulative" nature
of this quest. Likewise, scientists are still looking for a
satisfactory way to integrate general relativity with quantum
 mechanics.[9]



We cannot know in advance how far science will succeed, or that nature
is *constituted* as wholly law-like. This would represent a
"constitutive principle," a "cosmological"
knowledge claim which goes beyond any possible experiences. Instead,
the principle of seeking law-like unity represents a
"maxim" or *regulative principle* of reason
(A666/B694). Rather than determining what science must find, such
principles guide scientific practice.



Kant's account of science, and especially the role of
"teleological" or purposive judgment, is further developed
in the *Critique of the Power of Judgment*. See Guyer 1990,
Freudiger 1996, and Nuzzo 2005, as well as
 Kant's aesthetics and teleology
 (SS3). On Kant's account of science more generally, see
Wartenberg 1992, Buchdahl 1992, Friedman 1992b & 2013, Mudd 2016
and Breitenbach 2018. On reason and science, see Neiman 1994: Ch. 2
and Grier 2001: Ch. 8. The entry on
 Kant's philosophy of science
 considers Kant's view of the natural sciences, especially
physics.


### 1.3 The limits of reason



This is the best-known point, and is considered in detail in the entry
on
 Kant's critique of metaphysics.
 Kant demolishes a series of supposed proofs of the existence of God
("The Ideal of Pure Reason") and the soul ("The
Paralogisms").



He also demonstrates that it is equally possible to prove some
judgments about "world wholes" as it is to prove their
opposites. "The Antinomies" contain arguments that space
must be unbounded *and* that it must be bounded, that there
must be an absolutely first cause *and* that there cannot be
(the problem of freedom in the famous "Third
Antinomy").



These sections have always been regarded as the most convincing parts
of the book. Mendelssohn spoke for many of Kant's contemporaries
in calling him the "all-destroyer." These contradictions
show the limits of metaphysical knowledge.



Kant's intentions are not destructive, however. Philosophers
must understand the capacity that they rely on--that is,
*reason*
 (SS1.4).
 Theologians and metaphysicians have often claimed knowledge that
reason cannot deliver. This has led to empty battles that invite
skepticism about reason. After all, if reasoning leads to inconsistent
claims, how can we rely on it? By contrast, Kant aims to clear the
ground: reason has limits but it is not powerless; philosophical
reasoning can justify *some* claims. These include the claims
discussed above
 (SS1.1
 and
 SS1.2)
 as well as reason's practical role
 (SS2
 below). Gava & Willaschek (forthcoming) usefully stress this
aspect of the first *Critique*.



In the Doctrine of Method (the last, least-read part of the first
*Critique*) Kant refers to the biblical story of Babel
(*Genesis*, Ch. 11). The literal meaning of "Babel"
is "confusion"--God punished human beings'
attempt to build "a tower that would reach the heavens"
(A707/B735) by giving them many different languages. Since they were
unable to understand one another, they could no longer cooperate in
such hubristic
 ventures.[10]
 Again and again, reason returns to some simple ideas with towering
implications--the immortal soul, God, freedom. Worse, it cooks
 up[11]
 more or less convincing proofs of these.



Since these ideas reach beyond experience of a shared world, people
lack a shared way to test them. Perhaps they emptily repeat other
people's words without real understanding; quite possibly, they
come up with conflicting versions of these ideas. They talk past one
another; they might as well speak different languages. Most likely,
they will fall into conflict, or find peace only by submitting to an
unreasoned authority. In metaphysics, Kant refers to "the
ridiculous despotism of the schools"
 (Bxxxv).[12]
 In practical life, however, despotism is far from ridiculous: it is
the brutal last resort for securing order when people adopt
conflicting ideas and pursue conflicting goals. Kant often alludes to
Hobbes, who holds that peace is only possible if an unaccountable
sovereign "overawes" every member of
 society.[13]
 On many interpretations, Kantian reason aims to build intersubjective
order and avoid the dangers of Babel-like hubris, conflict and
despotism (Saner 1967, O'Neill 1989, Neiman 1994).



In the Preface to the second edition of the *Critique*, Kant
makes a famous claim: "I had to deny *knowledge* in order
to make room for *faith*" (Bxxx). Human beings cannot
have knowledge of the world as a whole. They cannot know entities that
transcend this world, such as the immortal soul or God. We cannot
experience these things through our senses; reason cannot supply such
knowledge. However, Kant argues that knowledge is *not* the
primary end of reason. Only *our role within the world*
"necessarily interests everyone" (A839n/B867n). Kant
rejects a "scholastic" or knowledge-oriented notion of
philosophy. Instead, he offers a "cosmic" or
world-oriented one (A838/B866; cf. Ypi 2021: Ch. 1 and Ferrarin
2015).



Kant proposes three questions that answer "all the interest of
my reason": "What can I know?" "What must I
do?" and "What may I hope?" (A805/B833). We have
seen his answer to the first question: we can only know the world as
revealed through the senses. Kant does not answer the second question
until the *Groundwork of the Metaphysics of Morals*, four years
later. (Arguably, he sees no need to answer the question in this form,
since he is confident that people have long known how they ought to
 act.[14])
 But the first *Critique* does include some observations on
hope--that is, faith in God and a future world. Kant argues that
knowledge of these things is not only impossible--it would also
corrupt practical reasoning. Rather than doing good for its own sake,
we would be motivated by external incentives--eternal punishment
and heavenly reward. Kant later calls this "heteronomy,"
as opposed to "autonomy"--our own personal commitment
to morality. Despite this, Kant argues that we have legitimate reasons
to hope for God and immortality. We must also have confidence in our
freedom to act morally. He connects and develops these claims in the
second *Critique*, as discussed below
 (SS2.3).



Since "reason" is a mental capacity, it may seem strange
to speak of it having "needs" or "interests."
The basic idea is that there are preconditions for successfully
exercising this capacity. For finite human beings, reason is not
transparent or infallible, as some rationalist philosophers seemed to
think. We may *think* we are reasoning, when actually we are
cooking up false rationalizations and self-deceptions. We may think we
are reasoning *well*, when we argue toward transcendent truths,
such as the existence of God or a future world. So reason has an
"interest" in appreciating its own limits, if it is to be
valid. As Kleingeld puts it, reason "needs to present itself to
itself in the process of gaining clarity about its own workings"
(1998a: 97)--above all, the principles that it must give to
itself. As the next section discusses, this means that Kant views
reason as essentially *self-reflexive*.


### 1.4 Reason's self-knowledge



The first *Critique* argues that there has, so far, been no
real progress in metaphysics. In the second edition Preface, Kant
proudly proclaims that his book has finally put metaphysics on
"the sure path of a science" (Bvii; cf. Axiii). So we
might ask: what is the relation of metaphysics--or philosophical
reasoning more generally--to areas of human enquiry that generate
certainty (geometry and mathematics) and expand knowledge (science in
general)?



Kant insists that mathematics cannot provide a model for philosophy.
"Mathematics gives the most resplendent example of pure reason
happily expanding itself without assistance from experience"
(A712/B740). But metaphysics cannot follow this
 course.[15]
 This is not simply a rhetorical point, since many of Kant's
predecessors had tried to do exactly this--Spinoza's
*Ethics* is one example, Christian Wolff's philosophy
another (see Gava 2018). Kant's basic argument is that
mathematicians are justified in *constructing* objects or
axioms a priori, because they can work with pure intuitions--like
a line or the form of a triangle, for example--rather than being
restricted only to the analysis of concepts. (See the entry on
 Kant's philosophy of mathematics.)
 Philosophers cannot follow this sort of procedure because they have
no right to assume any a priori intuitions or axioms about
metaphysical entities. Attempts that rely on such claims have only
produced "so many houses of cards" (A727/B755).



The empirical sciences also offer an unpromising model for
metaphysics. In the first place, Kant has argued that experience
cannot reveal metaphysical entities. We could never *know*, for
instance, that we are free: like everything else we can know, human
conduct is open to causal explanation. Second, experience cannot
generate the sort of necessity Kant associates with metaphysical
conclusions. Experience reveals *contingent* facts: it cannot
show that such-and-such *must* be the case. (Kant's
position here is complicated, since he also holds that scientific laws
have the metaphysical quality of necessity. Genuine laws are
*universal*, and not mere generalizations or rules of thumb.
But this metaphysical claim requires philosophical
justification--thus Kant's short book, *Metaphysical
Foundations of Natural Science* (1786). It is not discovered by
investigating the world.)



None of these points, however, deters Kant from using the imagery
of science and experiment to describe his own philosophical efforts.
These metaphors are especially prominent in the Preface to the second
edition of the *Critique*, where he writes:



> 
> Reason, in order to be taught by nature, must approach nature with its
> principles in one hand, according to which the agreement among
> appearances can count as laws, and, in the other hand, the experiment
> thought out in accord with these principles--in order to be
> instructed by nature not like a pupil, who has recited to him whatever
> the teacher wants to say, but like an appointed judge who compels
> witnesses to answer the questions he puts to them. (Bxiii)
> 



In other words, reason, as "[self-]appointed judge," does
not sit by and merely observe whatever comes along. It actively
proposes principled accounts of the phenomenon it
investigates--that is, law-like *hypotheses*. Then it
devises *experiments* to confirm or disprove these.



Kant sees grounds for optimism in this picture. The natural sciences
investigate the infinite scope of the empirical world. In contrast,
philosophy only considers "what reason brings forth entirely out
of itself... as soon as [its] common principle has been
discovered" (Axx). The Transcendental Dialectic of the first
*Critique* offers one application of this idea. Kant insists
that there are only three transcendental ideas--the thinking
subject (or soul), the world as a whole (the universe, or the entirety
of time and space), and a being of all beings (that is, God)
(A334/B391). As such, metaphysics is prone to three fundamental
mistakes, which spring "from the nature of reason" itself
(A339/B397; for more detail, see
 Kant's critique of metaphysics).
 Philosophers have made many efforts to reach these ultimate or
"unconditioned" objects. But these efforts always fail and
they always involve the same basic mistakes.



Whether or not Kant is correct in this, his own picture of
philosophical reasoning may seem puzzling. He suggests that reason
must conduct an experiment *upon itself*--an idea that is
hard to make sense of. In addition, although he talks about
reason's "common principle," he does not explain
what this may be.



Kant's "experiment" may be less puzzling if we
consider his hypothesis about everyday knowledge. He makes an analogy
with Copernicus's suggestion that the earth orbits the sun (Bxvi
f). Only if we take account of the earth's motion can we
understand what we see in the heavens: we need to appreciate how our
own (relative, moving) standpoint affects our observations.
Kant's parallel hypothesis is that our experience depends on the
standpoint and capacities of the observer. Our knowledge is not like
God's (albeit more confused and limited), as rationalist
philosophers sometimes suggested. Human knowledge has a structure of
its own, and its own limits too.



The first main sections of the *Critique* explain and defend
Kant's philosophical hypothesis. We must take account of our
human standpoint in order to explain the "a priori
structure" of our experience--for example, how it is
unified in a single order of space and time. The hypothesis is
supported partly because the main alternative fails. The
non-Copernican or non-Kantian view assumes a "single
standpoint." It does not distinguish between everyday knowledge
and metaphysical speculation "beyond the bounds of
experience" (Bxix). This leads to all sorts of problems. For
example, we might suppose that immortal souls could interact with
material objects and mortal human beings. Kant satirized this view in
his earlier book, *Dreams of a Spirit-Seer Elucidated by Dreams of
Metaphysics* (1766). His stated target was a religious visionary
called Emanuel Swedenborg. But the subtitle reveals Kant's
philosophical point: metaphysicians have been dreaming, too. They have
not appreciated the structure and limitations of the human standpoint.
So their arguments have created "an unavoidable conflict of
reason with itself" (Bxviii n). The "Antinomies"
mentioned above
 (SS1.3)
 explain the basic contradictions that result.



Even if Kant's arguments succeed, this still leaves awkward
questions about philosophical reasoning. Kant is not just concerned
with our capacities for everyday knowledge. As the book's title
says, he means to "critique" or examine reason. But
reasoning is the only means we have to do this. Does it make sense to
think of reason as criticizing or justifying itself? Might it even
point to something distinctive about our capacity to reason, that it
can examine itself in this way?



SS3
 below discusses the most thorough reply to these question in the
literature, by Onora O'Neill. To anticipate briefly:
Kant's metaphors--of reason's experiment upon itself,
as well as legal metaphors about testimony and
 examination[16]--point
 to a general problem: reason's *self-knowledge* (cf. Axi
f).



Kant assumes that we have a capacity to reason and that "to
philosophize [is] to exercise the talent of reason..."
(A838/B866). But since metaphysical reasoning has often gone wrong, he
cannot merely assert the supremacy of reason. As he exclaims in the
second edition Preface:



> 
> how little cause have we to place trust in our reason if in one of the
> most important parts of our desire for knowledge it does not merely
> forsake us but even entices us with delusions and in the end betrays
> us! (Bxv)
> 



Kant's question, then, is how we might *defend* reason
from various
 doubts[17]
 and how we might *discipline* reason without begging
questions--for instance, by invoking claims that are themselves
open to doubt (cf. O'Neill 1989: Ch. 1, 1992, 2004 & 2015).
This is the central task of *critique* (cf. Bxxxv):
reason's examination of itself. This should establish
reason's powers and limits. It should give an account of
reason's "common principle" and its authority.


## 2. Practical reason: morality and the primacy of pure practical reason



The first *Critique* only hints at the form of Kant's
moral theory would
 take.[18]
 The account of practical reason in the *Groundwork of the
Metaphysics of Morals* (1785) and *Critique of Practical
Reason* (1788) is radically new. Kant now presents the supreme
principle of practical reason--the Categorical Imperative. It is
an *imperative*: it represents a command for human beings, who
have needs and inclinations and are not perfectly rational. It is
*categorical* or unconditional: it must always guide our
action.



Kant's first formulation of this principle runs as follows:
"act only according to that maxim through which you can at the
same time will that it become a universal law" (4:421). (Kant
also gives other versions of this Imperative, which he sees as
equivalent: see
 Kant's Moral Philosophy,
 SS5-SS9.) Kant argues that this principle
is implicit in common human reasoning. Although it requires much
philosophical effort to articulate this principle, we rely on it
whenever we make moral judgments.



Alongside the Categorical Imperative, Kant also mentions the principle
of "hypothetical imperatives." As he puts it,
"Whoever wills the end also wills (in so far as reason has
decisive influence on his actions) the indispensably necessary means
to it that is in his control" (4:417; cf.
 5:19f[19]).
 Following Hume, many philosophers hold that practical reasoning is
essentially instrumental. In this case, practical reason only gives
conditional or "hypothetical" instructions. These have an
"if--then" structure. *If* someone has
particular ends or inclinations, *then* they should adopt
suitable means. (This is the idea of
 instrumental rationality;
 see also
 Kant's moral philosophy,
 SS4;
 Kant and Hume on morality,
 SS3.)



In Kant's view, this is just a matter of consistency. In his
terms, the principle of hypothetical imperatives is
*analytic*--to pursue an end *is* to employ
"the indispensably necessary means" (4:417; cf. Korsgaard
1997, Newton 2017, Pollok 2017: Ch. 8). Someone who fails to take up
necessary means is not, in fact, pursuing that end. At best, they are
hoping or wishing. At worst, they are involved in a practical
contradiction--a sure sign of irrationality. However, they can
overcome this contradiction by abandoning the end. Seen as mere
consistency, rationality cannot require more.



By contrast, the categorical imperative is *synthetic* (4:420,
447). It provides an end that every rational agent *must*
 adopt.[20]
 For Kant, the requirement to adopt universalizable maxims means that
we must respect all rational beings as
"ends-in-themselves" (4:428). For Kant, reason--and
only reason--is the source of this unconditional demand.


### 2.1 Freedom implies moral constraint: the Categorical Imperative



Alongside the derivation of his supreme moral principle, the most
difficult questions about Kant's view of practical reason center
on its relation to freedom. Although the broad outlines are
consistent, Kant's views on this topic seem to shift more than
other aspects of his critical thought. (See
 Kant's moral philosophy,
 SS10, for a brief sketch, and Allison 1990 for a full account.)
This section and the following
 SS2.2
 focus on Kant's radical claim that "freedom and
unconditional practical law [that is, the Categorical Imperative]
reciprocally imply each other" (5:29f). Freedom implies that
practical reason can be pure (non-instrumental, unconditional), and
hence that we are subject to the Categorical Imperative. Our
subjection to morality implies that we must be free.



Kant's argument that freedom implies the Categorical Imperative
is brief (see *Critique of Practical Reason*, 5:19-30).
If I am free to step back from my inclinations, those inclinations
cannot provide a definitive reason to act. Inclinations motivate, but
they do not compel. If someone asks me why I did something,
inclinations may *explain* my action--the action
helped me achieve something I wanted. But there is still an open
question: should I have acted on those inclinations?



An example may help here. Pushing you out of the way might satisfy my
desire to get to the front of the queue. The desire motivates me; we
might say that it provides a "reason for me." For other
people, my desire only *explains* my rudeness. It does not
supply others with a reason to endorse my behavior; it does not
*justify* my doing this. Compare the entry on
 reasons for action: justification, motivation, explanation:
 "Because I wanted to" may be a "motivating
reason" for me; it may be an "explanatory reason"
for other people. But it is not a "justificatory reason."
Kant accepts that desires are relevant to practical reasoning. His
claim is that they need not, and should not, determine our reasoning.
Simply put, my inclinations do not matter more than other
people's.



How can we find reasons that others should also accept? Kant claims
the only possibility is to look to "the mere *form* of
giving universal law" (5:27). There is, he says, "only a
single categorical imperative... *act only according to that
maxim through which you can at the same time will that it become a
universal law*" (4:421). This principle is categorical (no
if-s or but-s!); it is an imperative (we *must* follow it!).
When our action passes this test, we have done our duty. (In fact, the
first section of the *Groundwork* argues that this principle
follows just by analyzing the idea of duty. For more detailed
discussion see
 Kant's moral philosophy,
 SS2-SS5.)



Kant's emphasis on duty may sound old-fashioned; the idea of
justifying your conduct is more familiar, especially when we think
about practical *reasoning*. However, duty and justification
are inseparable. To claim that you have done your duty is to hold that
your actions are *justified*. Other people should endorse your
actions as the right thing to do. If they resent or frustrate your
actions, they are acting unreasonably--not you.



But why does Kant think that the "the mere *form* of
giving universal law" can capture the idea of duty, or reveal
what sort of action is justified?



Two points are central. First, the "form of... law"
refers to an overall policy or principle. Without this, my actions
would be random and make no sense, even to me. Second, Kant refers to
"giving universal law." Not just any policy or principle
will do. A justification is meant to provide other people with reasons
to endorse my conduct--common sense already tells us that many
policies won't do this. In fact, Kant argues that there is only
one. My policy must be to act on principles that everyone else can
also adopt. This is what he means by "universal law."



To give a better sense of what he means, Kant notes some other
principles that *seem* law-like, but go beyond the "mere
form of universal law"--and thereby fail to be justifiable.
In the second *Critique* (5:39ff), he discusses six such
candidates. Here is one example to illustrate.



Kant sees Epicurus as advocating the following policy: always follow
your inclinations, wherever they might lead. This might sound
law-like--it directs action, just as ordinary laws tell you to
pay your taxes or drive on the left. It may even sound like a law that
a free agent could adopt. While it deprives you of the freedom to act
in ways you *don't* want, it preserves something that
sounds much more worthwhile: the freedom to do whatever you
*do* want. However, it goes beyond the "form of
law" because it tells you to submit to merely subjective
factors--your inclinations, whatever they may be. This raises two
problems.



First, the policy is not actually law-like in practice--my
inclinations change and conflict with one another. For example,
short-term inclinations often undermine longer-term ones; often
it's impossible to get everything we wish for. This isn't
so much freedom, more a recipe for confusion. To act coherently, I
need to discipline and rank my inclinations. In other words, my will
would contradict itself if I tried only to follow inclinations. I
would not really have a will--in Kant's terms, there is a
contradiction in even *conceiving* this policy.



Second, it is not a policy that everyone can follow. If they did, the
results would be utterly chaotic; they would defeat anyone's
attempts to satisfy their inclinations. In Kant's terms, there
is a contradiction in *willing* such a policy. So far as we can
even imagine this, we see straightaway that each person's
willing would predictably fail.
 (Kant's moral philosophy,
 SS5, explores these two contradictions, in conception and in
willing.)



In more abstract terms: such a policy is unreasonable because it gives
authority to particular features of one particular agent--worse,
to features that aren't even stable or mutually consistent. In
Kant's words:



> 
> it is requisite to reason's lawgiving that it should need to
> presuppose only *itself*, because a rule is objectively and
> universally valid only when it holds without the contingent,
> subjective conditions that distinguish one rational being from
> another. (5:21)
> 



No principle is truly law-like unless it abstracts from an
agent's particular motivations and situation. Every other
possibility looks for substantive guidance from outside of reason
itself--just as a hypothetical imperative only guides you
*if* you already want to pursue that end. Kant refers to this
basic difficulty as *heteronomy*--that is, reasoning
directed from the outside. This external authority might be my
inclinations, or religious dogma, or even a political authority. But
it is merely assumed or imposed; it cannot justify itself.
Heteronomous reasoning does not address the fundamental problem: to
find principles that are entitled to guide everyone's acting and
thinking. In the word Kant has made so famous, reasoning must be
*autonomous*--that is, self-governed.



There have been many doubts whether the mere "form of law"
can really provide concrete guidance. In reply, Kantians argue that it
represents a substantial constraint: we must avoid all ways of
thinking and acting that cannot be followed by all. (For discussion
see *inter alia* O'Neill 1989: Ch. 5; Herman 1993;
Allison 1990: Ch. 10 SSII.) If this is true, then the autonomy of
reason reveals the positive sense of freedom at the heart of
Kant's practical philosophy (cf. Brandom 1979). That is, reason
enables us to act in ways that do *not* rely on
"contingent, subjective conditions that distinguish one rational
being from another" (5:21). It enables us to justify our conduct
to one another. It empowers us to build communities around principles
that everyone can share.


### 2.2 Moral constraint implies freedom: Kant's "fact of reason"



As well as claiming that freedom implies the Categorical Imperative,
Kant claims that moral obligation implies freedom.



Kant insists that "nothing in appearances can be explained by
the concept of freedom" (5:30). Morality, he claims,
"exists in the sensible world [the world as known through the
senses and by science] but without infringing on its laws"
(5:43). Every action counts as an event in the world of everyday
objects (or "appearances," in Kant's terminology).
We must take every event to be caused--in terms of everyday
explanations as well as in sciences such as physics and neuroscience.
Considered as events in the *objective* world, human actions do
not provide us with a warrant for freedom.



Instead, it is to our consciousness or *subjectivity* that Kant
turns:



> 
> Ask [someone] whether, if his prince demanded, on pain of...
> immediate execution, that he give false testimony against an honorable
> man who the prince would like to destroy under a plausible pretext, he
> would consider it possible to overcome his love of life... He
> would perhaps not venture to assert whether he would do it or not, but
> he must admit without hesitation that it would be possible for him. He
> judges, therefore, that he can do something because he is aware that
> he ought to do it and cognizes freedom within him, which, without the
> moral law, would have remained unknown to him. (5:30; cf. 5:155f)
> 



As Kant also says, "the moral law, and with it practical reason,
[have] come in and forced this concept [freedom] upon us"
(5:30). In the next section, Kant refers to this as a "fact of
reason": "Consciousness of this fundamental law may be
called a fact of reason because one cannot reason it out from
antecedent data of reason" (5:31; cf. 5: 6, 42f, 47, 55, 91,
104).



This "fact" has caused considerable controversy, for
several reasons. Kant is not altogether clear about what he takes this
fact to demonstrate. He has argued that facts about human beings
cannot provide the basis for morality--morality must be given a
priori, that is, independently of experience. Further, Kant speaks of
"cognizing the moral law." But he is well aware that his
own account of this "law" is quite new. Not least, this
"fact, as it were" does not feature in his earlier
treatise, the *Groundwork*, and does not appear again.



One school of thought--which includes many influential Kant
scholars, and is sympathetically represented in Allison 1990 (Chs. 12
and 13)--sees a fundamental change in Kant's thought here.
Part III of the *Groundwork* seems to give a
"deduction" (justification) of freedom. But in the second
*Critique* Kant realizes that his own premises do not allow
such an argument. So he stops argument short by appealing to a
supposedly indubitable fact.



Other commentators emphasize the clear continuities between the two
books. In particular, Kant relies on common moral consciousness in
both. Lukow 1993 emphasizes the parallel between
*Achtung* ("respect" or "reverence" for
morality) and the fact of reason. Kant refers to respect in all his
ethical writings: it is the only feeling "*self-wrought*
by a rational concept [= the moral law]" (4:401n). As such, it
clearly resembles what he now calls "the sole fact of pure
reason" (5:31). (See also O'Neill 2002 and Timmermann
2010.) Moreover, Kant uses the Latin word *factum*. Arguably,
this is better translated as *deed* rather than fact. In other
words, Kant points to an *act* of reason as well as the
*fact* that it creates (Kleingeld 2010).



In any case, Kant's line of thought in the long passage quoted
above is fairly clear: We all (most of us) recognize that there are
situations where we ought to do something, even though it will cost us
dearly. In other words, we feel that we are subject to an
unconditional moral imperative. When we acknowledge this
"ought," we show our belief in the possibility of acting
on it. This shows us to be free--not merely in the negative sense
that we could go against our inclinations, but also in the positive
sense that we can do our duty (4:446). Only our moral awareness
reveals this freedom.



Clearly, this line of thought is not immune to criticism. For example,
our feeling of moral constraint might be explained in terms of a
Freudian super-ego. But it shows why Kant thought that moral
awareness--unlike any other sort of experience--gives us a
practical certainty of our freedom. While this is not knowledge in the
empirical or scientific sense, it provides a basis for practical
reasoning: "a fact [*Faktum*] in which pure reason in us
proves itself actually practical" (5:42). And if Kant is right
that only the Categorical Imperative reveals our capacity to act in
ways that we can justify to others, then we can see why he claims,
"freedom and unconditional practical law reciprocally imply each
other" (5:29f).


### 2.3 The primacy of (pure) practical reason



Kant does not give a complete account of the relation between
practical reason and theoretical reason. However, the second
*Critique* does include an important section, "On the
primacy of pure practical reason in its connection with speculative
reason" (5:119-121). (See Gardner 2006 and Willaschek
2010.)



We have already seen two reasons why Kant must give some priority to
"pure practical reason." First, assume that Kant is
correct--against rationalist philosophers--that theoretical
reason cannot have insight into the supersensible. This means that
reason cannot have access to any transcendent authority (such as God
on many traditional conceptions) that might command thought or
action.



Second, Kant says that practical reason can either be
"pure"--i.e., "proceed from a priori
principles"--or it can proceed "from empirical
determining grounds" (5:90). Desires and inclinations are
empirical, for Kant. Reasoning that helps us fulfill them is
"hypothetical"--it applies only if you have those
desires. For Kant, merely following your desires represents
"heteronomy," especially when they conflict with morality.
On this view, reason is just an *instrument* to help you
fulfill your desires. As Hume put it, in an especially horrible
metaphor, reason would be "the slave of the passions"
(*Treatise*, 2.3.3.4). It cannot issue its own
 commands.[21]



These two points suggest that neither theoretical nor instrumental
reasoning can supply authoritative reasons for action. If there are
such reasons--as Kant's "fact of reason"
supposes--then *only* pure practical reason can supply
them.



Now, however, Kant makes a stronger claim. He argues that pure
practical reason has "primacy" even on the home territory
of theoretical reason. That is, pure practical reason should guide
some of our *beliefs*, as well as our actions.



Kant defines primacy as "the prerogative of the interest of one
insofar as the interests of others is subordinated to it"
(5:119). He gives at least three considerations to support this
"prerogative" of practical
 reason.[22]



One point just reflects Kant's talk of reason's
"interests." "[A]ll interest," says Kant,
"is ultimately practical and even that of speculative reason is
only conditional and is complete in practical use alone"
(5:121). In this sense, to accept the metaphor of
"interests" is to see reasoning as ultimately practical.
(Cf. Kant's notion of philosophy as "world-oriented"
rather than
 "scholastic"--SS1.3
 above. See also Neiman 1994: Ch. 3; Kleingeld 1998a; Rauscher
1998.)



Second, practical reason can be "pure" or independent from
"pathological conditions," that is, our inclinations. As
just noted, Kant holds that practical reason can be "pure"
or "a priori." That is, it need not be governed by
our inclinations. By contrast, theoretical reason goes wrong when
it seeks knowledge by itself--for example, in merely
"speculative" proofs that God exists. Theoretical
reasoning can only gain knowledge through sensibility and
understanding. Practical reason is independent in a way that
theoretical reasoning can never be.



A third line of thought is less abstract, and rests on the specific
interests that Kant attributes to these two forms of reasoning. When
their interests conflict, we must decide which are to come first.



The "interest" of theoretical reason consists in expanding
our knowledge and avoiding error. As Kant has argued, reason can help
broaden scientific knowledge, but it cannot justify speculative
metaphysical claims. However, practical reason has a conflicting
interest--in beliefs that lie beyond the bounds of knowledge.
Kant writes:



> 
> But if pure reason of itself can be and really is practical, as the
> consciousness of the moral law proves it to be [cf.
>  SS2.2
>  on the "fact of reason"], it is still only one and the
> same reason which, whether from a theoretical or a practical
> perspective, judges according to a priori principles; it is then clear
> that, even if from the first [theoretical] perspective its capacity
> does not extend to establishing certain propositions affirmatively,
> although they do not contradict it, *as soon as these same
> propositions belong inseparably to the practical interest* of pure
> reason[, then theoretical reason] must accept them. (5:121)
> 



In other words, even if it is finally "one and the same
reason," reason seems divided. Used practically, reason has an
interest in "certain propositions"--specifically, the
reality of freedom, immortality and God. But theoretical reasoning
says these cannot be proven. The practical interest has greater
weight, because it is rooted in morality.



Here, Kant introduces the idea of a "postulate." This is
"a *theoretical* proposition, though one not demonstrable
as such, insofar as it is attached inseparably to an a priori
unconditionally valid *practical* law" (5:122). For Kant,
there is only one unconditional law: the Categorical Imperative or
"moral law." He claims that three postulates are
"attached" to it: "immortality, freedom considered
positively (as the causality of a being insofar as it belongs to the
intelligible world), and the existence of God" (5:132).



Freedom has a special status--as discussed in
 SS2.2,
 the second *Critique*'s "fact of reason" is
meant to establish this. But why does he think we must
"postulate" God and
 immortality?[23]
 (See also the entry on
 Kant's philosophy of religion.)



As we have seen, Kant holds that we should act out of
"respect" for morality--for the sake of duty.
Morality must have priority over my inclinations. (Remember:
inclinations can *motivate* and *explain* actions, but
they are not enough to *justify* action.) This means
that there is no guarantee that acting well will lead to my own
happiness, or even other people's. This creates a
conflict--in Kant's terms, a
"dialectic"--between happiness and morality. While
morality is the only *unconditional* good for human beings,
Kant does not deny that happiness is important. It is the natural and
necessary end of every human being (cf. 4:415); duty requires us to
make others' happiness our
 end.[24]



Kant expresses the combined importance of morality and happiness as
"the highest
 good":[25]



> 
> *virtue* (as worthiness to be happy) is the *supreme
> condition* of whatever can even seem to us desirable and hence of
> all our pursuit of happiness... and is therefore the
> *supreme* good. But it is not the whole and complete good for
> finite rational beings; for this, *happiness* is also required,
> and that not merely in the partial eyes of a person who makes himself
> an end, but even in the judgment of an impartial reason [in other
> words, this is not about my subjective wish to be happy, but rather an
> objective judgment that happiness is the natural end for human beings,
> just as goodness is our moral end--GW]... happiness
> distributed in exact proportion to morality (as the worth of a person
> and his worthiness to be happy) constitutes the *highest good*
> of a possible world. (5:110)
> 



The argument here is bold but dubious. We must think of moral activity
as resulting in happiness. Yet human agency cannot achieve this:
"I [or even all of us acting together--GW] cannot hope to
produce this [highest good] except by the harmony of my will with that
of a holy and beneficent author of the world" (5:129). So, Kant
argues, we must "postulate" that God exists. We must also
postulate immortality. Only this enables us to hope that we will
finally come close to virtue, hence become worthy of happiness.


## 3. The unity of theoretical and practical reason



We have seen one link between theoretical and practical reason. In
answer to the question, "What may I hope?" Kant claims
that practical reason has "primacy." Theoretical reason
may accept the postulates of God, freedom and immortality "as a
foreign possession handed over to it" (5:120). Theoretical
reason can only show that these things are not impossible. Our
practical interests--at root, our moral obligations--demand
that we believe in them. To cite Kant's famous words again:
"I had to deny *knowledge* in order to make room for
*faith*" (Bxxx).



Kant's arguments for freedom are more extensive and compelling.
His arguments for God and immortality find little favor among
contemporary authors. Perhaps the strongest support comes from Jens
Timmermann, who claims: "the principle that unifies the spheres
of theoretical and practical reason... is the assumption of a
wise and benevolent God who has created a teleological world that
coheres with morality" (2009: 197). (On teleology in the
*Critique of the Power of Judgment*, see Guyer 1989, Freudiger
1996;
 Kant's aesthetics and teleology,
 SS3. Outside that work, see also Wood 1970, Kleingeld 1998b.)



Even if Kant did believe this--in his weaker moments,
 perhaps[26]--such
 a position lacks wider philosophical resonance. Most contemporary
philosophers assume that the world does not "harmonize
with" morality in this way--or at any rate, that it is a
human task to foster such harmony, for example, by building decent
societies and just institutions. Reath 1988 argues that Kant sometimes
deploys a more defensible, "secular" or this-worldly,
notion of the highest good. Kleingeld 1995 and Guyer 2000a & 2000b
also offer interesting discussions.



Onora O'Neill has made the leading attempt to uncover the unity
of Kantian reason and to relate it to contemporary philosophical
concerns (1989 and subsequent essays). This section will focus on her
central claim concerning the unifying role of the Categorical
Imperative, and the main bases for this claim in Kant's texts.
O'Neill's interpretation of Kantian reason enjoys
considerable respect among Kant scholars, although there are
relatively few critical responses to her work. Engstrom 1992 and Wood
1992 offer early reviews; Westphal 2011 and Cohen 2014, 2018 take up
her account, as does Bagnoli 2022: Ch. 4; see further
 constructivism in metaethics,
 SS2 (especially
 SS2.3).[27]


### 3.1 Reason's "common principle"



In the original Preface to the first *Critique*, Kant suggested
that reason has a "common principle": "Nothing here
can escape us, because what reason brings forth entirely out of itself
cannot be hidden, but is brought to light by reason itself as soon as
reason's common principle (*gemeinschaftliches Prinzip*)
has been discovered" (Axx). Unfortunately, neither edition of
the *Critique* considers what this principle might be.



Kant also raises this principle in his works on practical reason,
without giving a clear account. In the Preface to the
*Groundwork*, Kant explains why the book is not entitled a
*Critique of Pure Practical Reason*:



> 
> [A critique of pure practical reason] is not of such utmost necessity
> as [a critique of pure theoretical reason], since human reason, even
> in the commonest understanding, can easily be brought to a high
> measure of correctness and accuracy in moral matters, whereas in its
> theoretical but pure use it is totally and entirely dialectical [i.e.,
> a source of illusion]... I require that the critique of a pure
> practical reason, if it is to be complete, also be able to present its
> unity with speculative reason in a common principle; because in the
> end there can be only one and the same reason, which must differ
> merely in its application. (4:391)
> 



In the second *Critique*, Kant compares the book's
structure with the first *Critique* and comments: "such
comparisons [are] gratifying; for they rightly occasion the
expectation of being able some day to attain insight into the unity of
the whole rational faculty (theoretical as well as practical) and to
derive everything from one principle--the undeniable need of
human reason, which finds complete satisfaction only in a complete
systematic unity of its cognitions" (5:91). Kant's tone is
confident. But once again, he has postponed an account of "the
unity of the whole rational faculty." (Prauss 1981 argues that
Kant failed to achieve this insight, in part because he did not
appreciate how cognitive success is a fundamentally practical goal.
Forster 1992 discusses Kant's reflections on this topic in
his final manuscript, the *Opus Postumum*.)



However, as Onora O'Neill points out in a celebrated essay
(1989: Ch. 1), Kant's claims about practical reason imply a
further claim about reason's "common principle."
Kant has argued that the Categorical Imperative is the supreme
principle of practical reason. He has also argued that practical
reason has primacy over theoretical reason. It follows, therefore,
that the Categorical Imperative is the supreme principle of
reason.



To be sure, Kant never states this conclusion explicitly. But there
are reasons for thinking that this ought to have been his view, and in
some places he comes very close to this
 claim.[28]



The clearest passage is a footnote (!) in Kant's essay,
"What is it to Orient Oneself in Thinking?" (1786):



> 
> To make use of one's own reason means no more than to ask
> oneself, whenever one is supposed to assume something, whether one
> could find it feasible to make the ground or the rule on which one
> assumes it into a universal principle for the use of reason. (8:146n)
> 



This bears an obvious parallel with the first formulation of the
Categorical Imperative--"act only according to that maxim
through which you can at the same time will that it become a universal
law" (4:421). Kant now says: *think* only in accordance
with that maxim that could be a universal
 law.[29]



To rephrase the point: Thinking and judging are activities. If the
Categorical Imperative is truly "categorical" then it
applies to *all* our activities--"theoretical"
as well as "practical."



Other strands of Kant's thought support this interpretation. The
most important are his "maxims of reason" and his account
of the "public use of reason."


### 3.2 The maxims of reason



Kant sets out three "maxims of reason" or "maxims of
common human understanding." They appear twice in his published
writings, in relation to both acting and
 thinking.[30]
 These maxims are closely related to the Categorical Imperative. (For
discussion, see O'Neill 1989: Ch. 2 & 1992, and Neiman 1994:
Ch. 5.)



In his last published work, the *Anthropology* (1798), Kant
presents the maxims in a *practical* context, as guidelines for
achieving some degree of wisdom:



> 
> *Wisdom*, as the idea of a practical use of reason that
> conforms perfectly with the law, is no doubt too much to demand of
> human beings. But also, not even the slightest degree of wisdom can be
> poured into a man by others; rather he must bring it forth from
> himself. The precept for reaching it contains three leading maxims:
> (1) Think for oneself, (2) Think into the place of the other [person]
> (in communication with human beings), (3) Always think consistently
> with oneself. (7:200; cf. 228f)
> 



The maxims also appear in the *Critique of the Power of
Judgment* (1790). Here, Kant relates them to the
*theoretical* use of reason. A famous section describes human
beings' capacity for a *sensus communis* or
"community sense":



> 
> a faculty for judging that... takes account (a priori) of
> everyone else's way of representing in thought, in order *as
> it were* to hold its judgment up to human reason as a whole and
> thereby avoid the illusion which, from subjective private conditions
> that could easily be held to be objective, would have a detrimental
> influence on the judgment. (5:293)
> 



That is, the maxims are precepts for judging in accordance with
"reason as a whole" and avoiding the distortions that
arise from "subjective private conditions."



Kant describes (1) thinking for oneself as the maxim of
*unprejudiced* thought; its opposite is passivity in thought,
leading to prejudice and superstition. Kant also labels this
"heteronomy," clearly recalling his interpretation of
reason and morality in terms of autonomy
 (4:433).[31]
 (2) To think in the place of everyone else is the maxim of
*enlarged* or *broad-minded* thought. And (3) always to
think in accord with oneself is the maxim of *consistent*
thought (5:294).



The last maxim may sound more straightforward, but Kant emphasizes its
difficulty. Consistency "can only be achieved through the
combination of the first two and after frequent observance of them has
made them automatic" (5:295). Consistency does not just involve
getting rid of obvious contradictions in explicit beliefs. It also
requires consistency with regard to the implications of our
beliefs--and these are often not apparent to us. To achieve this
sort of *law-likeness in thought* requires genuine attempts (1)
to judge for oneself and the determination (2) to expose one's
judgments to the scrutiny of others. In other words, a person must try
to make themselves: (1) really the *author* of their judgments;
(2) *accountable* to others; and (3) *consistent* in
their claims. We must *take responsibility* for our
judgments.



The maxims support the thesis that theoretical and practical reasoning
have a unified structure. In particular, they flesh out the
implications of the Categorical Imperative. The imperative demands
respect for "the mere form of law"--consistency,
law-likeness, and autonomy in thought and action. Equally, it demands
respect for other people as fellow reasoners and
"ends-in-themselves" (Russell 2020, Lenczewska 2021). As a
matter of thought, reasoning requires us to discipline our judgments
so that others can follow them. As a matter of practical wisdom,
reasoning requires us to discipline our actions so that others can
adopt the same guiding principles.


### 3.3 The public use of reason



Anyone concerned with Kantian reason and politics must consider his
famous essay, "What is Enlightenment?" (1784). Among many
discussions, see O'Neill 1989: Ch. 2, 1990 & 2015: Ch. 3;
Velkley 1989; Deligiorgi 2005; Patrone 2008.



As we have seen, Kant's first maxim of reason is to think for
oneself. He says this is the way to achieve "liberation from
superstition," and equates this with "enlightenment"
(5:294). His second maxim asks us to "think into the place of
others." Communication is essential for this. As he writes:
"...how much and how correctly would we think if we did not
communicate with others to whom we communicate our thoughts, and who
communicate theirs with
 us!"[32]



In "What is Enlightenment?" Kant places both maxims in a
political context. He demands that we "have the courage to use
our *own* reason":



> 
> *Enlightenment is the human being's emergence from his
> self-incurred immaturity. Immaturity* is the inability to use
> one's own understanding [=
>  reason[33]]
>  without the guidance of another. This immaturity is
> *self-incurred* if its cause is not lack of understanding, but
> lack of resolution and courage to use it without the guidance of
> another. *Sapere aude!* [Dare to be wise!] Have courage to make
> use of your *own* understanding [= reason]! is thus the motto
> of enlightenment. (8:35)
> 



Kant's main concern is not with individuals, however.
Enlightenment is a collective endeavor. It therefore has cultural and
political aspects. For enlightenment, Kant says, "nothing is
required but... the least harmful... freedom: namely,
freedom to make *public use* of one's reason in all
matters" (8:36).



This "least harmful freedom" is not the freedom to act
politically. It is not even freedom of speech in the now-familiar
sense that covers many forms of personal expression. Instead, it is
the freedom to reason "*as a scholar* before the entire
public of the *world of readers*" (8:37).



As we might expect, Kant contrasts public reasoning with private
reasoning. However, the distinction has an unfamiliar meaning. For
Kant, employees reason privately. He mentions civil servants, military
officers, or clerics in an established church. In contemporary terms,
these are all more or less "public" roles. However, these
employees are bound by the policies, procedures and dictates of their
organization. They reason about how to act--but only to judge the
best way of fulfilling tasks laid down by their employers.



We can draw a parallel with an individual who reasons
instrumentally--about the best way to satisfy their inclinations,
for example. Instrumental reason decides the best means to achieve
pre-given ends. In Kant's terms, it is
heteronomous--directed by a source outside reason itself.



Similarly, these employees are directed by state or church. Their
offices may be "public," but their reasoning is
*private*--deprived of full publicity. The employee
"carries out another's commission" (8:38). His
reasoning is not free: it is "impermissible to argue; instead,
one must obey" (8:37). The employee does not abandon his own
moral judgment--Kant is careful to add that he must resign if he
could not "in conscience [continue to] hold his office"
(8:38). But within that office, he acts as a "passive
member" (8:37) of the commonwealth.



By contrast, the public use of reason is active and autonomous. It
does not follow directions; it is accountable to all. When a person
reasons publicly, he thinks for himself and speaks as a member of
"the society of citizens of the world" (8:37). Outside of
his post, an employee may reason freely, and may even criticize
government policies or religious teachings.



The same applies to the ordinary citizen. He must obey--for
example, by paying taxes--and may not speak in a way that
encourages others to disobey. But he "does not act against the
duty of a citizen when, as a scholar, he publicly expresses his
thoughts about the inappropriateness or even injustice of
[governmental] decrees" (8:38).



This creates a stark contrast between roles: the obedient citizen or
employee, as opposed to the "scholar" who makes
"public use of reason." But Kant sees no inconsistency.
Peaceful co-existence requires that citizens do not undermine, let
alone rebel against, existing governments--that risks chaos and
even civil war. In Kant's words, "morally practical reason
pronounces [an] irresistible *veto: there is to be no
war*" (6:354). As citizens of a given state, we must act as
political institutions demand. As citizens of the world, however, we
may criticize those demands and advocate
 reform.[34]



Kant's views invite criticism. In his day, only a few citizens
could read and publish their views; hardly any women could. So his
position may seem elitist and sexist. It implies a gulf between theory
and practice--between what employees and citizens may believe
ought to be done, and what they must do ("obey"!).
"The least harmful freedom" may seem not just harmless but
powerless. Freedom to publish is only a small part of the freedoms
essential to modern democracy. Hence Kant's views may seem
antiquated. Certainly, they fall far short of democratic citizenship
as we understand it.
 Kant's social and political philosophy
 (SS4 and SS6) discusses these issues.



For this entry, the key point is this. Kant equates reason with full
publicity. "To use one's own reason" is to make a
sincere attempt to address *all* "citizens of the
world." Judgments and principles are *only* reasonable to
the extent that they can be accepted by all. Our actions must respect
existing boundaries and governments, because the duty to pursue and
maintain peace is categorical. But reason itself aspires to
universality. So as citizens of the world, we have another categorical
duty: to see ourselves as accountable to every human being, and hence
to improve existing institutions so that they recognize the claims of
all. For reasoning to be *fully* public, citizens and
non-citizens, rulers and ruled must discuss as equals--despite
their inequalities in practice.



A famous passage from the *Critique of Pure Reason* expresses
this idea, linking reason, critique, and autonomy:



> 
> Reason must subject itself to critique in all its undertakings, and
> cannot restrict the freedom of critique through any prohibition
> without damaging itself and drawing upon itself a disadvantageous
> suspicion. For there is nothing so important because of its utility,
> nothing so holy, that it may be exempted from this searching review
> and inspection, which knows no respect for persons [i.e. no person
> bears more authority than any other--GW]. On this freedom rests
> the very existence of reason, which has no dictatorial authority, but
> whose claim is never anything more than the agreement of free
> citizens, each of whom must be able to express his reservations,
> indeed even his veto, without holding back. (A738f/B766f, translation
> modified)
> 



In the term used by several contemporary Kantians, this procedure
*constitutes* reason. (See Herman 2007: Ch. 10, Korsgaard 2008,
Reath 2013, Bagnoli 2017; Moller 2020 focuses on Kant's
legal metaphors.) Reason is the only unconditional (that is,
non-heteronomous) authority for thinking and acting.


### 4. Concluding remarks



No one doubts that belief and action are subject to demands of
rationality. Theoretical reasoning aims at knowledge of the world
around us (perhaps also knowledge of ourselves). Practical reasoning
aims to make a difference in the world (perhaps also a difference in
ourselves).



This distinction hides a larger question: what *is* reason,
 exactly?[35]
 Rationality clearly involves the capacity to make logical and causal
inferences: to draw out the consequences of our beliefs; to judge what
consequences are likely to follow from different possibilities for
action. Rationality clearly requires consistency. Our beliefs should
not contradict one another, or have contradictory implications. We
should use means that support our goals, rather than defeat them. We
should avoid contradictory
 goals.[36]
 Beyond this, however, it is not obvious that "reason" is
a unitary capacity or that "rationality" is a unitary
requirement. If truth seems self-evident as a criterion of thinking,
the criteria for rational action are much harder to make out.



If we accept the basic lesson of the first *Critique*, it is
even harder to discern the criteria of rational action. Against
rationalist philosophers, Kant argues that theoretical reason cannot
discover realities beyond empirical experience. (Recall the literal
meaning of metaphysics: *beyond* the physical world.) So we
cannot know any sort of moral authority, be it God or revelation, that
tells us how to think and act. Traditionally, truth and goodness stood
together. Mendelssohn called Kant the "all-destroyer"
partly because his critique undercut this traditional pairing.
Experience can disclose (empirical) truth, but not goodness.



Perhaps, then, we can only reason practically in order to satisfy
pre-existing feelings or goals. These factors can motivate us to
reasoning, but they are not themselves open to rational justification.
Hume's empiricist account of morality illustrates the point.
Reason is "the slave of the passions." We can only hope
that our fellow human beings feel sympathy, and will put reason to
work in its cause. Although we may disapprove, we cannot say it is
*irrational* to feel or pursue more anti-social
"passions" such as spite or envy or pride. In Hume's
own words, "'Tis not contrary to reason to prefer the
destruction of the whole world to the scratching of my finger...
'Tis as little contrary to reason to prefer even my own
acknowledg'd lesser good to my greater..."
(*Treatise* 2.3.3.6).



Since Kant has cut off (theoretical) knowledge of goodness,
doesn't consistency drive him to similar skepticism? Since his
account is structured by a distinction between theoretical and
practical reason, how can he show their unity?



This entry has suggested that Kant's account of reason is
constructive (not skeptical) and unified (not divided). In thought and
in action, reasoners must look for principles that others can also
adopt:



> 
> To make use of one's own reason means no more than to ask
> oneself, whenever one is supposed to assume something, whether one
> could find it feasible to make the ground or the rule on which one
> assumes it into a universal principle for the use of reason.
> ("What is it to Orient Oneself in Thinking?" 8:146n)
> 



In theoretical enquiry, we seek knowledge of the world and explore the
limits of knowledge; we think and philosophize. We hope to reach
judgments that are valid for all and draw on principles that all can
rely on.



In practical reasoning, we decide how to act, individually and with
others. Kant often emphasizes duty, but this entry has stressed the
related idea of *reasoning as justification*. If an action is
justified, other people should endorse it, rather than resent or
criticize it. If their circumstances are similar, they should act
similarly. Even if their circumstances differ, the same underlying
principles should guide their actions. If we can share principles, we
can justify our conduct and find a basis for cooperation.



O'Neill (2000) situates the Kantian account against three
alternatives. (i) The *instrumental* account of reason remains
popular. Following Hume, rational choice theory and
 consequentialism
 see practical reasoning as a tool. Reason works out how to achieve
given ends (cf.
 practical reason,
 SS4;
 reasons for action: internal vs. external;
 instrumental rationality;
 philosophy of economics,
 SS5). (ii) The *communitarian* account sees reason as
embedded within shared traditions. Rationality is what a historical
community takes it to be--for example, its usual modes of
argument and accepted authorities (cf. MacIntyre 1988;
 communitarianism).
 (iii) The *perfectionist* account sees reason as an individual
capacity to discern moral truth or goodness. This view resembles the
rationalism which Kant opposed, except that it stresses divine
revelation or moral intuition, rather than abstract thought (cf.
 moral non-naturalism,
 SS3).



Arguably, all three accounts fail in providing reasoned justification
to some audiences. (i) The instrumental reasoner cannot justify their
action to people who do not endorse their goal. Instrumental reasons
*explain* action in terms of the actor's desires or ends.
They can make sense of the action, but they do not give the audience
reasons to endorse it. Explanation is not justification. (ii) For the
communitarian, their tradition defines what beliefs and practices
count as reasonable. If someone disagrees, such a reasoner can say
little except: this is how I have been taught; this is how we do
things round here. This sort of reasoning cannot justify itself to
outsiders, nor to insiders who dissent. (Recall Kant's free
citizens who "must be able to express [their] reservations,
indeed even [their] veto.") (iii) The perfectionist believes
that they can intuit how it is good to act or to be. But what can they
say to someone who has different "intuitions," or
none--except perhaps to condemn them as blind or confused?



On the interpretations advanced by Saner, O'Neill, Neiman and
others, Kant was aware of these options and rejected them all. We saw
above
 (SS1.4)
 that Kant characterizes reason in terms of a self-reflexive
procedure. Reason is autonomous--that is, it rejects all external
authorities. Reason has authority only because it rests on on-going
critique. Openness to critique means giving up any mode of thinking or
acting that cannot be adopted by all.



To put the same point in more concrete terms: Kant ties reasoning to
justification. His account stresses the limits of many everyday
justifications. Consider the modes of reasoning just mentioned. (i)
"Because I wanted to" is sometimes a perfectly good
reason--*so long as* I don't infringe on
others' legitimate claims. (ii) "Because that's how
we usually do things" is sometimes a perfectly good
reason--*so long as* our practices don't endanger or
humiliate or exploit. (iii) "Because it felt like the right
thing to do" may be a perfectly good reason--*so long
as* my "intuitions" reflect a moral sensibility that
fits my circumstances.



The Kantian point is this: Don't forget the proviso,
"*so long as...*" In the wrong circumstances,
desires, customs and intuitions can lead us astray. Reasoning is
incomplete *unless* we can also justify giving weight to the
desires, customs or intuitions that make something seem reasonable to
us. By contrast, reasoning is complete when it meets the following
test: Everyone could think and act on the same principles that now
guide my thought and action. To judge whether our thinking and acting
meet this demanding test, we must consider objections from different
points of view. As Kant says, each person "must be able to
express [their] reservations." This is an on-going process, with
no fixed or final endpoint. Only this active process can make
reasoning fully "public," "autonomous" and
"universal(izable)."



The Kantian view does not assume that we are hemmed in by our
interests and inclinations--as many instrumental accounts do. It
does not ask us to rely on what others already accept--as the
communitarian account does. It does not suppose that we can intuit
what everyone should accept--as the perfectionist account does.
Not least, it does not suppose that reason can teach us about God or
any other moral authority beyond everyday experience--as many
rationalist philosophers supposed. Kant's account sees human
beings as able to step back from their inclinations, habits and
intuitions. Reasoning is not the voice of God, but rather a capacity
to heed the voices of those we share a world with. Kant asks us to use
this ability to seek principles that all can think and live by, and to
organize our lives together on this basis.



This account depends on a particular interpretation of Kant's
texts. It is abstract, ambitious, and complex in its ramifications.
But if it succeeds, it captures two powerful aspects of Kant's
philosophy: a universalism that transcends self and community
boundaries; a modesty that respects the limits of human insight.