of our will; suppose that reason contained certain practical, original
principles a priori, which were absolutely impossible without this
presupposition; and suppose, at the same time, that speculative reason
had proved that liberty was incapable of being thought at all. It
would then follow that the moral presupposition must give way to the
speculative affirmation, the opposite of which involves an obvious
contradiction, and that liberty and, with it, morality must yield to
the mechanism of nature; for the negation of morality involves no
contradiction, except on the presupposition of liberty. Now morality
does not require the speculative cognition of liberty; it is enough
that I can think it, that its conception involves no contradiction,
that it does not interfere with the mechanism of nature. But even this
requirement we could not satisfy, if we had not learnt the twofold
sense in which things may be taken; and it is only in this way that
the doctrine of morality and the doctrine of nature are confined
within their proper limits. For this result, then, we are indebted
to a criticism which warns us of our unavoidable ignorance with regard
to things in themselves, and establishes the necessary limitation of
our theoretical cognition to mere phenomena.

[*Footnote: In order to cognize an object, I must be able to prove
its possibility, either from its reality as attested by experience,
or a priori, by means of reason. But I can think what I please, provided
only I do not contradict myself; that is, provided my conception is
a possible thought, though I may be unable to answer for the existence
of a corresponding object in the sum of possibilities. But something
more is required before I can attribute to such a conception objective
validity, that is real possibility--the other possibility being merely
logical. We are not, however, confined to theoretical sources of
cognition for the means of satisfying this additional requirement,