But, though all our knowledge begins with experience, it by no means follows that all arises out of experience.
For, on the contrary, it is quite possible that our empirical knowledge is a compound of that which we receive through impressions, and that which the faculty of cognition supplies from itself (sensuous impressions giving merely the occasion), an addition which we cannot distinguish from the original element given by sense, till long practice has made us attentive to, and skillful in separating it.

(Immanuel Kant "Critique of Pure Reason" Translated by J. M. D. Meiklejohn)