Kant has earned the great compliment of having detractors
who criticize him with great insight and ingenuity. German idealism, which
dominated nineteenth-century philosophy, finds its footing by attacking
Kant’s conception of things-in-themselves. Idealists such as Hegel
argue that there is something deeply suspicious about these mysterious
entities, which Kant claims are the source of our sensations while
claiming we can have no direct knowledge of them. Idealism jettisons
things-in-themselves and the whole noumenal realm, arguing instead
that reality consists primarily of mental phenomena. Analytic philosophy,
which is one of the leading schools of twentieth-century philosophy,
also gets its start through an attack on Kant. The logician Gottlob
Frege criticizes Kant for basing the analytic–synthetic distinction
on the subject-predicate form of grammar, which is not a necessary
feature of the logical structure of language or reality. Frege argues
that we should base the analytic–synthetic distinction on whether
we justify a given judgment by appealing to its logical form or
to empirical investigation and that, according to this distinction,
the category of the synthetic a priori becomes unnecessary. Kant
is only able to argue that geometry, for instance, relies on synthetic
a priori knowledge because he fails to distinguish between pure
geometry—the stuff of mathematical axioms and proofs—and empirical
geometry—the application of geometrical principles to science. Pure
geometry is a priori, but it is also analytic, since it is justified
according to logical principles alone. Empirical geometry is synthetic,
but it is also a posteriori, since we only learn from experience
what sort of geometry applies to the real world.
