244 Kant: A BiographyThough they make experiential knowledge possible, they might also make
other kinds of knowledge possible. Indeed, that they make purely meta¬
physical knowledge (of nonspatial and nontemporal objects) possible had
been a fundamental thesis of his Inaugural Dissertation. It is the task of
the Transcendental Deduction to show that the categories are necessary
for experiential knowledge and insufficient for knowledge of objects inde¬
pendent of space and time.
Kant tries to show that while the categories per se are independent of
experience, their use is necessarily restricted to spatio-temporal experience.
They are designed for us to think about objects of experience, or appear¬
ances. His Transcendental Deduction, one of the most difficult passages
in the first Critique, is essentially an attempt to establish this restriction on
our use of the categories. The details of Kant's arguments are admittedly
messy. It is not always clear what Kant means or how he takes himself to
have proved particular points. Nonetheless, the general strategy is clear.
Kant intends to show that the categories are possible concepts only inso¬
far as they make experience possible; when applied to anything that goes
beyond experience, they become merely empty words:
The Transcendental Deduction of all a priori concepts has thus a principle according
to which the whole inquiry must be directed, namely, that they must be recognized as
a priori conditions of the possibility of experience, whether of the intuition which is
to be met with in it or of the thought. Concepts which yield the objective ground of
the possibility of experience are for this very reason necessary (Ao.4=Bi26).
The Deduction shows that the categories, which, he claims, may be assumed
as given because of the Metaphysical Deduction, are necessarily presup¬
posed in any possible experience (which is also presupposed as given). Kant
intends to show that the use of the categories is justified insofar as it is re¬
lated to this experience.
This is one of the most important differences between the categories
or concepts of the understanding, and the ideas or concepts of reason, which
constitute Kant's third "form," or the forms of reason. The latter are
discussed in the second part of the Transcendental Logic, namely the Tran¬
scendental Dialectic. Ideas cannot form "the basis of any objectively valid
synthetic judgment," while "through concepts of understanding reason
does, indeed, establish secure principles, not however directly from con¬
cepts alone, but always only indirectly through relation of these concepts
to something altogether contingent, namely possible experience" (A737=
B765). Thus he can say that any principle of the understanding has "the