Leibnitz, anxious to escape the danger into which Descartes fell, of making the outward world purely phenomenal, an expression of unalterable thought, and also to escape the consequences of Locke's position that all knowledge originates in the senses, suggested that the understanding itself was independent of experience, that though it did not contain ideas like a vessel, it was entitled to be called a power of forming ideas, which have, as in mathematics, a character of necessary truths. These necessary laws of the understanding, which experience had no hand in creating, are, according to Leibnitz, the primordial conditions of human knowledge. Hume, taking Locke at his word, that all knowledge came from experience, that the mind was a passive recipient of impressions, with no independent intellectual substratum, reasoned that mind was a fiction; and taking Berkeley at his word that the outward world had no material existence, and no _apparent_ existence except to our perception, he reasoned that matter was a fiction. Mind and matter both being fictions, there could be no certain knowledge; truth was unattainable; ideas were illusions. The opposing schools of philosophers annihilated each other, and the result was scepticism. Hume started Kant on his long and severe course of investigation, the result of which was, that neither of the antagonist parties could sustain itself: that Descartes was wrong in asserting that such abstract ideas as causality, infinity, substance, time, space, are independent of experience, since without experience they would not exist, and experience takes from them form only; that Locke was wrong in asserting that all ideas originated in experience, and were resolvable into it, since the ideas of causality, substance, infinity and others certainly did not so originate, and were not thus resolvable. It is idle to dispute whether knowledge comes from one source or another--from without through sensation, or from within through intuition; the everlasting battle between idealism and realism, spiritualism and materialism, can never result in victory to either side. Mind and universe, intelligence and experience, suppose each other; neither alone is operative to produce knowledge. Knowledge is the product of their mutual co-operation. Mind does not originate ideas, neither does sensation impart them. Object and subject, sterile by themselves, become fruitful by conjunction. There are not two sources of knowledge, but one only, and that one is produced by the union of the two apparent opposites. Truth is the crystallization, so to speak, that results from the combined elements.