In sections 33 and 34 of the Prolegomena, Kant indicates what happens when we try to
apply the concepts of the understanding beyond the limits of sense.
There is indeed something seductive in our pure concepts of the understanding which tempts
us to a transcendent use--a use which transcends all possible experience.
Not only are our concepts of substance, of power, of action, of reality, and others,
quite independent of experience, containing nothing of sense appearance, and so apparently
applicable to things in themselves (noumena), but, what strengthens this conjecture, they
contain a necessity of determination in themselves, which experience never attains.
The concept of cause contains a rule according to which one state follows another necessarily;
but experience can only show us that one state of things often or, at most, commonly follows
another, and therefore affords neither strict universality nor necessity.
Hence concepts of the understanding seem to have a deeper meaning and content than can
be exhausted by their merely empirical use, and so the understanding inadvertently adds
for itself to the house of experience a much more extensive wing which it fills with nothing
but beings of thought, without ever observing that it has transgressed with its otherwise
legitimate concepts the bounds of their use.
Two important and even indispensable, though very dry, investigations therefore became
indispensable in the Critique of Pure Reason [viz., the two chapters "The Schematism of
the Pure Concepts of the Understanding" and "The Ground of the Distinction of All Objects
in General into Phenomena and Noumena"].
In the former there is shown that the senses furnish, not the pure concepts of the understanding
in concreto, but only the schema for their use, and that the object conformable to it
occurs only in experience (as the product of the understanding from materials of sensibility).
In the latter there is shown that, although our pure concepts of the understanding and
our principles are independent of experience, and despite the apparently greater sphere
of their use, still nothing whatever can be thought by them beyond the field of experience,
because they can do nothing but merely determine the logical form of the judgment with regard
to given intuitions.
But as there is no intuition at all beyond the field of sensibility, these pure concepts,
since they cannot possibly be exhibited in concreto, are void of all meaning; consequently
all these noumena, together with their sum total, the intelligible world, are nothing
but representations of a problem, the object of which in itself is quite possible but the
solution, from the nature of our understanding, totally impossible.
For our understanding is not a faculty of intuition but of the connection of given intuitions
in an experience.
Experience must therefore contain all the objects for our concepts; but beyond it no
concepts have any meaning, since no intuition can be subsumed under them.