In this manner, then, the a priori principles of two faculties of
the mind, the faculty of 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