The Critique of Practical Reason | Page 6

cognition and that of desire, would be found and
determined as to the conditions, extent, and limits of their use, and thus
a sure foundation be paid for a scientific system of philosophy, both
theoretic and practical.
Nothing worse could happen to these labours than that anyone should
make the unexpected discovery that there neither is, nor can be, any a
priori knowledge at all. But there is no danger of this. This would be
the same thing as if one sought to prove by reason that there is no
reason. For we only say that we know something by reason, when we
are conscious that we could have known it, even if it had not been
given to us in experience; hence rational knowledge and knowledge a
priori are one and the same. It is a clear contradiction to try to extract
necessity from a principle of experience (ex pumice aquam), and to try
by this to give a judgement true universality (without which there is no
rational inference, not even inference from analogy, which is at least a
presumed universality and objective necessity). To substitute subjective
necessity, that is, custom, for objective, which exists only in a priori
judgements, is to deny to reason the power of judging about the object,
i.e., of knowing it, and what belongs to it. It implies, for example, that
we must not say of something which often or always follows a certain
antecedent state that we can conclude from this to that (for this would
imply objective necessity and the notion of an a priori connexion), but
only that we may expect similar cases (just as animals do), that is that
we reject the notion of cause altogether as false and a mere delusion.
As to attempting to remedy this want of objective and consequently
universal validity by saying that we can see no ground for attributing
any other sort of knowledge to other rational beings, if this reasoning

were valid, our ignorance would do more for the enlargement of our
knowledge than all our meditation. For, then, on this very ground that
we have no knowledge of any other rational beings besides man, we
should have a right to suppose them to be of the same nature as we
know ourselves to be: that is, we should really know them. I omit to
mention that universal assent does not prove the objective validity of a
judgement (i.e., its validity as a cognition), and although this universal
assent should accidentally happen, it could furnish no proof of
agreement with the object; on the contrary, it is the objective validity
which alone constitutes the basis of a necessary universal consent.
{PREFACE ^paragraph 30}
Hume would be quite satisfied with this system of universal empiricism,
for, as is well known, he desired nothing more than that, instead of
ascribing any objective meaning to the necessity in the concept of cause,
a merely subjective one should be assumed, viz., custom, in order to
deny that reason could judge about God, freedom, and immortality; and
if once his principles were granted, he was certainly well able to deduce
his conclusions therefrom, with all logical coherence. But even Hume
did not make his empiricism so universal as to include mathematics. He
holds the principles of mathematics to be analytical; and if his were
correct, they would certainly be apodeictic also: but we could not infer
from this that reason has the faculty of forming apodeictic judgements
in philosophy also- that is to say, those which are synthetical
judgements, like the judgement of causality. But if we adopt a universal
empiricism, then mathematics will be included.
Now if this science is in contradiction with a reason that admits only
empirical principles, as it inevitably is in the antinomy in which
mathematics prove the infinite divisibility of space, which empiricism
cannot admit; then the greatest possible evidence of demonstration is in
manifest contradiction with the alleged conclusions from experience,
and we are driven to ask, like Cheselden's blind patient, "Which
deceives me, sight or touch?" (for empiricism is based on a necessity
felt, rationalism on a necessity seen). And thus universal empiricism
reveals itself as absolute scepticism. It is erroneous to attribute this in

such an unqualified sense to Hume, * since he left at least one certain
touchstone (which can only be found in a priori principles), although
experience consists not only of feelings, but also of judgements.

* Names that designate the followers of a sect have always been
accompanied with much injustice; just as if one said, "N is an Idealist."
For although he not only admits, but even insists, that our ideas of
external things have actual objects of external things corresponding to
them, yet he holds that the form of the intuition does not depend