What if knowledge be neither a dull submission to dictation from without nor an unexplained necessity of thought? What if it be a bold adventure, an experimental sally of a Will to live, to know and to control reality? What if its principles were frankly _risky_, and their truth had to be _desired_ before it was tested and assured? In a word, what if first principles were to begin with _postulates?_ Thus the way is paved from the new psychology to a new theory of knowledge. A third alternative to the banal dilemma of 'empiricism' or 'apriorism' suggests itself. The old _empiricist_ view, as typified by Mill, was that the mind had been impressed with all its principles, such as the truths of arithmetic, the axioms of geometry, and the law of causation, by an uncontradicted course of experience, until it generalized facts into 'laws,' and was enabled to predict a similar future with certainty. But this theory had really been exploded in advance by Hume. Facts do not _appear_ as causally connected, nor, if they did, would this guarantee that they will continue to do so in the future. The continuum of experience, we may add, is not _given_ as a series of arithmetical units or geometrical equalities, unless we deliberately measure it out in accordance with mathematical principles. Empiricism thus gives no real account of the scientific rational order of the world. But does it follow from the failure of empiricism that apriorism is true? This has always been assumed, and held to dispense rationalist philosophers from giving any direct and positive proof that these principles are _a priori_ truths. But manifestly their procedure is logically far from cogent. If a third explanation can be thought of, it will _not_ follow that apriorism is true. All that follows is that _something_ has to be assumed before experience proves it. What that something is, and whence it comes, remains an open question. Moreover, apriorism has _not_ escaped from the empirical doubt about the future. Even granted that facts now conform to the necessities of our thoughts, why should they so comport themselves for ever? Let us, therefore, try a compromise, which ignores neither that which we bring to experience (like empiricism), nor that which we gain from experience (like apriorism). This compromise is effected by the doctrine of postulation. For though a postulate proceeds from us, and is meant to guide thought in anticipating facts, it yet allows the facts to test and mould it; so that its working modifies, expands, or restricts its demands, and fits it to meet the exigencies of experience, and permits, also, a certain reinterpretation of the previous 'facts' in order to conform them to the postulate.