This is sections 27 and 28 from Kant's Prolegomena to Any Future Metaphysics,
from the Second Part of the Main Transcendental Question.
Now we are prepared to remove Hume's doubt.
He justly maintains that we cannot comprehend by reason the possibility of causality, that
is, of the reference of the existence of one thing to the existence of another which is
necessitated by the former.
I add that we comprehend just as little the concept of subsistence, that is, the necessity
that at the foundation of the existence of things there lies a subject which cannot itself
be a predicate of any other thing; nay, we cannot even form a concept of the possibility
of such a thing (though we can point out examples of its use in experience).
The very same incomprehensibility affects the community of things, as we cannot comprehend
how from the state of one thing an inference to the state of quite another thing beyond
it, and vice versa, can be drawn, and how substances which have each their own separate
existence should depend upon one another necessarily.
But I am very far from holding these concepts to be derived merely from experience, and
the necessity represented in them to be fictitious and a mere illusion produced in us by long habit.
On the contrary, I have amply shown that they and the principles derived from them are firmly
established a priori before all experience and have their undoubted objective rightness,
though only with regard to experience.
Though I have no conception of such a connection of things in themselves, how they can either
exist as substances, or act as causes, or stand in community with others (as parts of
a real whole) and I can just as little think such properties in appearances as such (because
those concepts contain nothing that lies in the appearances, but only what the understanding
alone must think), we have yet a concept of such a connection of representations in our
understanding and in judgments generally.
This is the concept that representations belong in one sort of judgments as subject in relation
to predicates; in another as ground in relation to consequent; and, in a third, as parts which
constitute together a total possible cognition.
Further we know a priori that without considering the representation of an object as determined
with regard to one or the other of these moments, we can have no valid cognition of the object;
and, if we should occupy ourselves with the object in itself, there is not a single possible
attribute by which I could know that it is determined with regard to one or the other
of these moments, that is, belonged under the concept of substance, or of cause, or
(in relation to other substances) of community, for I have no conception of the possibility
of such a connection of existence.
But the question is not how things in themselves but how the empirical cognition of things
is determined, as regards the above moments of judgments in general, that is, how things,
as objects of experience, can and must be subsumed under these concepts of the understanding.
And then it is clear that I completely comprehend, not only the possibility, but also the necessity,
of subsuming all appearances under these concepts, that is, of using them as principles of the
possibility of experience.