A postulate thus fully meets the demands of apriorism. It is 'universal' in claim, because it is convenient and economical to make a rule carry as far as it will go; and it is 'necessary,' because all fresh facts are on principle subjected to it, in the hope that they will support and illustrate it. Yet a postulate can never be accused of being a mere sophistication, or a bar to the progress of knowledge, because it is always willing to submit to verification in the course of fresh experience, and can always be reconstructed or abandoned, should it cease to edify. A long and successful course of service raises a postulate to the dignity of an 'axiom'--_i.e._, a principle which it is incredible anyone should think worth disputing--whereas repeated failure in application degrades it to the position of a prejudice--_i.e._, an _a priori_ opinion which is always belied by its consequences. 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.