Critique of Pure Reason - Immanuel Kant

Guyer, P., and Wood, A., (eds.), Critique of Pure Reason, Cambridge: Cambridge University Press. 1998.

Excerpt:

Experience is no doubt the first product of our understanding, while employed in fashioning the raw material of our sensations. It is therefore our first instruction, and in its progress so rich in new lessons that the chain of all future generations will never be in want of new information that may be gathered on that field. Nevertheless, experience is by no means the only field to which our understanding can be confined. Experience tells us what is, but not that it must be necessarily as it is, and not otherwise. It therefore never gives us any really general truths, and our reason, which is particularly anxious for that class of knowledge, is roused by it rather than satisfied. General truths, which at the same time bear the character of an inward necessity, must be independent of experience—clear and certain by themselves. They are therefore called knowledge a priori, while what is simply taken from experience is said to be, in ordinary parlance, known a posteriori or empirically only.

Now it appears, and this is extremely curious, that even with our experiences different kinds of knowledge are mixed up, which must have their origin a priori, and which perhaps serve only to produce a certain connection between our sensuous representations. For even if we remove from experience everything that belongs to the senses, there remain nevertheless certain original concepts, and certain judgments derived from them, which must have had their origin entirely a priori, and independent of all experience, because it is owing to them that we are able, or imagine we are able, to predicate more of the objects of our senses than can be learnt from mere experience, and that our propositions contain real generality and strict necessity, such as mere empirical knowledge can never supply.

But what is still more extraordinary is this, that certain kinds of knowledge leave the field of all possible experience, and seem to enlarge the sphere of our judgments beyond the limits of experience by means of concepts to which experience can never supply any corresponding objects.