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of the Postulates. The interesting argument here is the Refutation of
Idealism, which Kant added to the B edition, along with a long foot-
note in the B edition Preface at Bxxxix“xli. As previously mentioned,
this argument responds to Descartes™s view that self-knowledge is
more certain than knowledge of external objects. Kant inserted this
proof, one of his key arguments against skepticism, in the middle
of his analysis of actuality. In spite of its location, the argument is
signi¬cant enough to stand on its own. Here I shall discuss Kant™s
general claims about the modal categories. The next section examines
the Refutation.
Analytic of Principles II 187
Judgments about real possibilities are governed by the postulate
that objects conform to the formal conditions of experience in gen-
eral, that is, to conditions of synthesis required for empirical cognition
(A220/B267). These conditions include both the forms of intuition,
and the categories defended previously. Hence for a state to be a
possible object of experience, it must ¬rst be located in the global
space-time of human intuition. Moreover, it must be both exten-
sively and intensively measurable (Axioms and Anticipations), and
governed by the principles of substance and causality (Analogies).
Clearly these are stronger constraints than the mere notion of logical
possibility expressed in the principle of non-contradiction. Kant says
the impossibility of a ¬gure enclosed between two straight lines “rests
not on the concept in itself, but on its construction in space” (A220“
1/B268). Given our form of spatial intuition, such a plane ¬gure is
not a possible object of experience. Similarly, invented concepts not
derived from the formal conditions of experience have no a priori
possibility.
Because assertions about the actual make stronger claims, they
depend on not only formal but also material conditions of experi-
ence, namely sensation. At A225/B270 Kant says this does not require
“immediate perception of the object itself” but rather “its connection
with some actual perception in accordance with the analogies of expe-
rience.” That is, one can assert the existence of a state of affairs that
is not immediately perceived, as long as it is connected by laws with
what is given in intuition. This permits us to assert the existence of
theoretical or unobserved entities such as electrons, dinosaurs, and
so on. Kant uses the example of a magnetic ¬eld: “Thus we cognize
the existence of a magnetic matter penetrating all bodies from the
perception of attracted iron ¬lings, although an immediate percep-
tion of this matter is impossible for us given the constitution of our
organs” (A226/B274). And in the Antinomies he says:

That there may be inhabitants in the moon, although no one has ever
perceived them, must certainly be admitted. This, however, only means that
in the possible advance of experience we may encounter them. For everything
is real which stands in connection with a perception in accordance with the
laws of empirical advance. (A493/B521)
Analytic of Principles II
188
This postulate also provides a criterion for distinguishing between
dreams and waking experience, since dream states are not integrated
into experience by causal laws.
Finally, the concept of empirical necessity applies only to states per-
ceived as following from other states according to causal laws. As Kant
puts it at A226/B280, real necessity is neither logical necessity, nor
the formal necessity of a valid inference (Kant™s logical form of neces-
sity), nor absolute metaphysical necessity. Instead it is a hypothetical
or material necessity of a state of affairs, given certain conditions,
according to a universal law. Consequently, real necessity attaches
not to substances, but only to their states: “Hence we cognize only
the necessity of effects in nature, the causes of which are given to
us . . . and even in this it does not hold of the existence of things,
as substances, since these can never be regarded as empirical effects,
or as something that happens and arises” (A227/B280). No substance
exists necessarily; as the Second Analogy shows, empirical necessity
attaches only to states of substances.
Finally Kant explains why he calls the modal principles postulates.
Although ˜postulate™ sometimes means a proposition assumed with-
out justi¬cation, this is not Kant™s de¬nition. As pure principles of the
understanding, the Postulates express “the synthesis through which we
¬rst give ourselves an object and generate its concept” (A234/B287).
Here Kant is thinking of propositions describing the construction
of ¬gures in space. These mathematical postulates cannot be proved
“since the procedure that it demands is precisely that through which
we ¬rst generate the concept of such a ¬gure.” Similarly, the Pos-
tulates of Empirical Thought cannot be proved since they do not
add content to the concept of an object, but only indicate how it
“is combined with the cognitive power” (A235/B287). In expressing a
priori conditions for judging real states of affairs, the Postulates make
it possible to construct a coherent system of empirical cognition.

6 . th e ref utati on of i de al i s m
Kant inserts the Refutation in the discussion of actuality, since he
intends to show that we must have actual knowledge of the exter-
nal world. Along with the revised Transcendental Deduction, this is a
major change in the B edition, added to clarify Kant™s idealism. In the
Analytic of Principles II 189
A edition Kant tackled the problem of the external world in the fourth
paralogism. There he claimed that transcendental idealism solves
the problem by showing that external objects “are merely appear-
ances, hence also nothing other than a species of my representations”
(A370“1). It is no wonder many readers confused his position with
empirical idealism, according to which physical objects are merely
collections of perceptions. The Refutation also makes an important
correction to Kant™s treatment of space and time. Whereas the Aes-
thetic treats outer and inner sense as parallel, the Refutation estab-
lishes the priority of outer sense to inner sense.
The argument is aimed against empirical idealism, “the theory that
declares the existence of objects in space outside us to be either merely
doubtful and indemonstrable, or else false and impossible” (B274).
Kant attributes the ¬rst variant, which he calls problematic ideal-
ism, to Descartes. The second he labels dogmatic idealism, which he
attributes to Berkeley, who claims that the ideas of mind-independent
space and matter are incoherent.37 Kant agrees with Berkeley if one
takes space to pertain to things in themselves, “for then it, along with
everything for which it serves as a condition, is a non-entity” (B274).
Since space is neither a substance nor a property of substances, it
could have no clear metaphysical status as a thing in itself. But the
conclusions that space is merely a form of intuition and not a thing in
itself refute dogmatic idealism in two ways. First, they show that space
and spatial things are possible as appearances; second, they prove that
space is a necessary condition for experience of particulars distinct
from the subject.
These arguments do not, however, address the possibility that
whereas experience may actually be temporal, it may only seem to be
spatial. Put another way, perhaps only inner sense is real, and outer
sense is imaginary. This is Descartes™s position in the second Medita-
tion, where he argues that his mental states are immediately knowable
and certain, while experience of physical things could be illusory. Not
until the sixth Meditation can he argue for the existence of physical
reality, based on his proofs of the existence of God. Although he con-
cludes that physical objects exist, Descartes maintains this knowledge

37 This discussion is drawn largely from my article, “On Kant™s Proof of the Existence of
Material Objects.”
Analytic of Principles II
190
is based on inference from perception, guaranteed by God™s benevo-
lence. Thus knowledge of the external world is in principle less certain
than self-knowledge.
In the Refutation, Kant will prove that knowledge of the exter-
nal world is just as certain as knowledge of one™s mental states. His
strategy is ad hominem: that is, Descartes could not have certain and
immediate knowledge of his mind unless he also had certain and
immediate knowledge of spatial objects. Now we must be clear on
what Kant is claiming about “outer” objects. According to transcen-
dental idealism, both the objects of outer sense and the self known
through inner sense are merely appearances and not things in them-
selves. Although Descartes was a transcendental realist, the issue here
is not transcendental realism vs. idealism, but the certainty of physical
knowledge compared to knowledge of the self. In the cogito, Descartes
claims certain knowledge of his thoughts while doubting that exter-
nal objects exist. In the order of experience, knowledge of the self is
prior to knowledge of physical objects. Regarding evidence, the exter-
nal world is not known directly, but only by inference from what is
directly perceived.
In a long footnote to the B edition Preface, Kant says it is a “scandal
of philosophy and universal human reason that the existence of things
outside us . . . should have to be assumed merely on faith” (Bxxxix).
The only way to prove that external objects are (empirically) real is
as a condition of inner experience. Hence the thesis to be proved is:
“The mere, but empirically determined, consciousness of my own
existence proves the existence of objects in space outside me.” In
Kant™s terms, the possibility of determinate inner sense presupposes
immediate awareness of objects through outer sense.
His proof at B275“6 (with an emendation at Bxxxix) consists of
¬ve steps:38
1. Descartes™s premise: “I am conscious of my existence as determined
in time” (B275). Descartes™s claim involves at least two capacities:
¬rst, to judge concerning any two mental states that they are both
mine; and second, to recognize the order in which such states occur
in consciousness.
38 Here I follow the accounts in Strawson, The Bounds of Sense, 125“8; Gochnauer, “Kant™s
Refutation of Idealism”; and Allison, Kant™s Transcendental Idealism, 297“304.
Analytic of Principles II 191
2. The First Analogy principle: “All time-determination presupposes
something persistent in perception” (B275). As we saw, the First
Analogy argues that determining objective temporal intervals pre-
supposes the existence of substance enduring through changes of
state. But since the proof establishes nothing about the nature of
these substances, Kant must show here that they are spatial.
3. The third step is stated in the B edition Preface this way:
But this persisting element cannot be an intuition in me. For all the
determining grounds of my existence that can be encountered in me are
representations, and as such they themselves need something persisting
distinct from them, in relation to which their change, and thus my exis-
tence in the time in which they change, can be determined. (Bxxxix)
Here Kant rules out for the substantial basis in perception both
aspects of the self, the representations and the thinking thing that
has them. Representations cannot qualify, since the issue is precisely
how I locate my representations in time. On the other hand, the
thing serving as the permanent substratum cannot be the thinking
self, since the only awareness of the self given in perception just is
of its temporary states. This is why Kant says there is no perma-
nent representation in intuition, that even representation of the
permanent is itself transitory (Bxli and B291“2).
4. Therefore the permanent must be “a thing outside me” and not “the
mere representation of a thing outside me” (B276). By a “thing
outside me” Kant means ¬rst, something numerically distinct from
the thinking self and its representations. But the thesis leaves no
doubt that this thing must be in space; its otherness guarantees its
physical nature.
5. Therefore determinate experience of myself as a particular thinker
proves that I immediately perceive physical objects; only by means
of this awareness can I know myself as the owner of my represen-
tations.
This argument has never been taken seriously. In particular,
commentators raise three issues: ¬rst, why the enduring objects
required to know oneself must be spatial;39 second, how the argument

39 Included among these objectors are Allison, Kant™s Transcendental Idealism, 303, and Ameriks,
Kant™s Theory of Mind, 121“2.
Analytic of Principles II
192
guarantees that these objects exist as opposed to being merely
imagined;40 and third, in what sense experience of spatial things
is immediate, especially given the role of concepts in experiencing
objects.41 Here I shall respond to these three objections.

A. Why enduring objects must be spatial
Kant™s strategy depends on what Descartes claims to know about
himself. This includes awareness of ¬rst, a succession of temporary
representations; second, the enduring self that has them; and third,
the coexistence of this permanent self with its changing representa-
tions. Thus Descartes claims certain and direct knowledge of things
having all three temporal characteristics: duration, succession, and
coexistence.
Now Kant does not dispute Descartes™s belief that he intuits some-
thing enduring. What he questions are Descartes™s identi¬cation of
the object as the thinking self, and his claim to intuit this self by the
intellect. The Aesthetic has shown that human intuition of existing
particulars, including themselves, is sensible. Moreover, as the Tran-
scendental Deduction shows, the ˜I think™ of transcendental apper-
ception is a purely formal consciousness and cannot represent the
particular self: “in the synthetic original unity of apperception, I am
conscious of myself, not as I appear to myself, nor as I am in myself,
but only that I am. This representation is a thinking, not an intuit-
ing” (B157“8). Descartes can recognize himself as a particular thinker
with his particular thoughts only through inner sense. The cogito con-
fuses the ˜I think™ of transcendental apperception with cognition of
the empirical self.
Moreover, intuition of the self in time presents only the succession
of mental states and not the permanent thinker who has them. As
Kant remarks at A107, consciousness of oneself “in internal percep-
tion is merely empirical, forever variable; it can provide no standing
or abiding self.” At Bxli Kant distinguishes between the represen-
tation of something permanent and a permanent representation. In

40 This point is raised by Broad, Kant, 198, and Allison, Kant™s Transcendental Idealism, 301“3.
41 For discussions of this point see Guyer, “Kant™s Intentions in the Refutation of Idealism,”
279“83, and Paton, Kant™s Metaphysic of Experience, 2:282“3.
Analytic of Principles II 193
fact, “there is no persistent intuition to be found in inner sense”
(B292). Everything given in inner intuition is merely transitory and
successive. Since distinct parts of time exist only successively, their
occupants must exist one after the other. Consequently, when I intuit
my representations as “in me,” the only temporal feature I am given is
succession. The formal features of inner intuition cannot provide the
awareness of duration and coexistence required to recognize oneself
as their owner.
Now as Descartes recognizes, in addition to their formal reality, rep-
resentations also have an objective reality: they represent some thing
to the thinker. So if the formal features of representations cannot
provide consciousness of myself as a persisting thing, then this aware-
ness must be achieved through the objects represented. Therefore I
must intuit through my representations something permanent, dis-
tinct from my mental states. In the Preface, Kant says this permanent
must be “a thing distinct from all my representations and external,
the existence of which is necessarily included in the determination
of my own existence . . . which could not take place even as inner if it
were not simultaneously (in part) outer” (Bxli). The “outer” part can
only be the reality presented in intuition, since representations are,
considered formally, “in me.” The objective reality of my immediate
consciousness must include things outside me where “outside” means
numerically distinct from my mental states.
Kant maintains that the only way to perceive an object as other than
myself is to locate it in space. The key is the contrast between space, as
a framework of permanent, coexisting locations, and time. Because
distinct parts of time exist successively, no temporal location, and
hence no occupant of a merely temporal location, is re-identi¬able
through time. But spatial locations exist non-successively. Although
both space and time as wholes are permanent, space alone is deter-
mined as permanent (B291). Unlike time, space can be divided into
numerically distinct, coexisting parts. It is the nature of distinct spa-
tial locations that they exist permanently. Consequently, each spatial
location, and hence its occupant, can in principle be re-identi¬ed
from one time to another. The permanence of coexisting parts of
space makes possible our consciousness of permanent and distinct
objects coexisting throughout our transitory perceptions. The cru-
cial features of objectivity “ independence of mere representation
Analytic of Principles II
194
and re-identi¬ability through time “ are possible only through the
permanence of space.

B. Why space of our experience cannot be merely imaginary
The above argument also explains why we can be certain that space is
real and not merely imaginary. Like many of his predecessors, Kant
believes that the ability to imagine external objects presupposes per-
ception of them. He says this at B276“7n: “in order for us even to
imagine something as external, i.e., to exhibit it to sense in intuition,
we must already have an outer sense, and by this means immediately
distinguish the mere receptivity of an outer intuition from the spon-
taneity that characterizes every imagining.” Unfortunately this cannot
be the reason that outer sense could not be merely imaginary, since it
begs the question. The real reply concerns what experience would be
like if space and spatial objects were merely imaginary. A merely imagi-
nary object is one that exists only through the subject™s representing, as
in dream states, hallucinations, and after-images. The objects of these
states do not in fact exist independently of their representation by the
subject. Since representations are only temporary, a merely imaginary
space and its occupants would last only as long as each representa-
tion of them. That is, each new representation would present a new,
numerically distinct spatial framework. If there were no continuity
of the spatial framework from one representation to another, there
could be no consciousness of enduring, continuous existence in time.
We could not recognize our passing states as thoughts, nor ourselves
as thinking things. The empirical reality of space is guaranteed by its
permanence. In being re-identi¬able through time, space and spatial
objects exhibit their independence of momentary representations,
including mere imaginings. To claim that one can imagine a per-
manent space is to erase the distinction between real and imaginary
space.
This is why Kant says, “For even merely to imagine an outer sense
would itself annihilate the faculty of intuition, which is to be deter-
mined through the imagination” (B277). Notice that not just outer
intuition, but the very faculty of intuition, would be annihilated
were we to lack outer sense. Intuition is the means by which we are
immediately related to objects (A19/B33). And objects are things that
Analytic of Principles II 195
correspond to and yet are distinct from our representations of them
(A104). But only outer sense presents awareness of enduring objects
distinct from the subject. The Cartesian hypothesis that I can know
my thinking self and merely imagine spatial things is not a possible
state of affairs after all.

C. The immediacy of spatial perception
The Refutation attempts to refute two Cartesian views: ¬rst, that self-
knowledge is prior to knowledge of the external world; and second,
that knowledge of physical reality is based on inference. Descartes
claims that only self-knowledge is immediate as both given directly in
consciousness, and as ¬rst-order consciousness of existence. To appre-
ciate Kant™s argument, we need to specify two distinctions between
immediate and mediate knowledge, the ¬rst concerning the evidence
justifying a belief, the second concerning the order of knowledge. Let us
designate “the immediate1 ” as a judgment non-inferentially justi¬ed
by intuition, as contrasted with one requiring an inference from that
data. Let us de¬ne “the immediate2 ” as original consciousness of exis-
tence, as opposed to consciousness derived from it. Now although
these two senses are closely linked, they are not equivalent if one
allows for a second-order or derivative consciousness that is given in
intuition. In that case a belief can be known “immediately1 ” (suf¬-
cient evidence is available in intuition) but nevertheless be “mediate2 ”
(not original consciousness of existence). This is in fact Kant™s view
of self-knowledge.
Kant argues in the Refutation that the permanent substratum of
objective time-determination must be spatial, that one must per-
ceive material objects in order to know oneself as a thinking sub-
ject. But the Aesthetic has shown that space is a pure form of intu-
ition, where intuition is the means by which objects are immediately
given to the subject. When Kant concludes that spatial experience is
immediate, he means that the abiding spatial framework presented in
intuition provides suf¬cient evidence for our belief in material exis-
tence. Unlike Hume, Kant does not identify the content of a belief
with its evidential basis, since for Kant all perception of particulars
(including space and time) also requires concepts of the understand-
ing. But concepts do not establish the existence of their objects: “In
Analytic of Principles II
196
the mere concept of a thing no characteristic of its existence can be
encountered at all” (A225/B272). If existential beliefs are not based on
concepts, then there are only two ways such beliefs can be justi¬ed:
either directly by intuition or indirectly by inference from it. But
if knowledge of external reality is guaranteed by the intuition of
space, then the Cartesians are wrong to claim that such knowledge is
only inferential. Thus Kant can conclude that knowledge of space is
immediate1 .
Now Kant agrees with Descartes that self-knowledge through inner
sense is also directly given, and hence immediate1 . Because intuition
provides direct awareness of both external and internal existence, Kant
says in the second edition Preface: “I am just as certainly conscious
that there are things outside me, which are in relation to my sense,
as I am conscious that I myself exist as determined in time” (Bxli).
Kant diverges most sharply from Descartes with respect to the order of
consciousness: the Refutation shows that perception of spatial objects
is epistemically prior to knowledge of the self. Thus ¬rst-order objec-
tive consciousness “ the immediate2 “ must be of physical reality.
It is from things outside us, Kant notes, that “we derive the whole
material of knowledge, even for our inner sense” (Bxxxix). And in the
Aesthetic: “the representations of outer sense make up the proper
material with which we occupy our mind” (B67). That outer sense
supplies the proper material of experience implies that inner sense
presupposes outer sense.
For Kant, awareness of our mental states is a second-order or
derivative consciousness, produced not by inference, but by re¬‚ection.
When he says “inner experience itself is consequently only mediate
and possible only through outer experience” (B277), he means that
self-awareness is mediate2 , that it requires prior awareness of external
objects. But this is consistent with the idea that both kinds of reality,
given in sensible intuition, are known immediately1 . In the Refuta-
tion, Kant argues that had Descartes no experience as a person in a
world of physical objects, he could never have discovered his thinking
self at all.
Taken together, the Analogies and the Refutation also refute
Descartes™s dualism, for they entail that there could be no purely
mental substance. Not only are the fundamental objects of experi-
ence spatial, but all substance must be spatial, and hence corporeal.
Since the criterion of substance is action, and only physical actions
Analytic of Principles II 197
are perceivable, the only entities that can count as substantial for Kant
are physical objects. This also implies that the empirical self, known
as an individual distinct from others, must be embodied. Even one™s
“mental” states must be located in space in two senses: ¬rst, they must
belong to a self who is a physical object, and second, the objects we
intuit through them must be physical. As we shall see in chapter 8,
Kant explicitly criticizes Descartes™s notion of the substantial soul in
the Paralogisms of the Transcendental Dialectic.

7. ka nt™s respon s e to ske pt ici s m

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