A ‘postulate’ thus differs essentially from the ‘a priori truth’ by its dependence upon the will, by its being the product of a free choice.  We have always to select the assumptions upon which we mean to act in our commerce with reality.  We select the rules upon which we go, and we select the ‘facts’ by which we claim to support our rules, stripping them of all the ‘irrelevant’ details involved by their position in the flux of happenings.  Thus we emphasize that side of things which fits in with our expectations, until the facts are ‘faked’ sufficiently to figure as ‘cases’ of our ‘law.’  Postulation and the verifying of postulates is thus a process of reciprocal discrimination and selection.  The postulate once formulated, we seek in the flux for confirmations of it, and thus construct a system of ‘facts’ which are relative to it; that is how the postulate reacts upon experience.  If, on the other hand, this process of selection is unfruitful, and the confirmations of our rule turn out infinitesimal, we alter the rule; and thus the ‘facts’ in the case reject the postulate.

This continuous process of selection and rejection of ‘principles’ and ‘facts’ has, as we have said, a thoroughly biological tinge.  The fitness of a postulate to survive is being continually tested.  It springs in the first place from a human hope that events may be systematized in a certain way, and it endures so long as it enables men to deal with them in that way.  If it fails, the formation of fresh ideals and fresh hypotheses is demanded; but that which causes one postulate to prevail over another is always the satisfaction which, if successful, it promises to some need or desire.  Thus ‘thought’ is everywhere inspired by ‘will.’  It is an instrument, the most potent man has found, whereby he brings about a harmony with his environment.  This harmony is always something of a compromise.  We postulate conformity between Nature and one of our ideals.  We usually desire more than we can get, but insist on all that Nature can concede.

Causation serves as a good example.  Experience as it first comes to us is a mere flood of happenings, with no distinction between causal and casual sequences.  Clearly our whole ability to control our life, or even to continue it, demands that we should predict what happens, and guide our actions accordingly.  We therefore postulate a right to dissect the flux, to fit together selected series without reference to the rest.  Thus, a systematic network of natural ‘laws’ is slowly knit together, and chaos visibly transforms itself into scientific order.  The postulation of ‘causes’ is verified by its success.  Moreover, it is to be noted that to this postulate there is no alternative.  A belief that all events are casual would be scientifically worthless.  So is a doctrine (still popular among philosophers) that the only true ‘cause’ is the total universe at one moment, the only true ‘effect,’ the whole of reality at the next.  For that is merely to reinstate the given chaos science tried to analyse, and to forbid us to make selections from it.  It would make prediction wholly vain, and entangle truth in a totality of things which is unique at every instant, and never can recur.