The new psychology of James was bound to produce a new theory of knowledge, and though it did not actually explore this problem, it contained several valuable suggestions upon the subject. For instance, in a brief passage discussing 'The Relations of Belief and Will,' James pointed out that belief is essentially an attitude of the will towards an idea, adding that in order to acquire a belief 'we need only in cold blood act as if the thing in question were real, and keep acting as if it were real, and it will infallibly end by growing into such a connection with our life that it will become real' (ii., p. 321). This passage is an outline of the doctrine of 'The Will to Believe,' which he was afterwards to develop so forcibly.

Again, in his last chapter, James criticized the doctrine of Spencer that all the principles of thought, all its general truths and axioms, were derived from impressions of the external world. He argued, on the other hand, that such ways of looking at phenomena must originate in the mind, and be prior to the experience which confirms them. Without digging further into the character of this mental contribution to knowledge, James contented himself with the suggestion that the use of these axiomatic principles might be construed in Darwinian style as a 'variation' surviving by its fitness, thus introducing into his account of mental process the important idea that thinking might be tested by its vital value.

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.

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.

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.

The principles of mathematics are as clearly postulates. In Euclidean geometry we assume definitions of 'points,' 'lines,' 'surfaces,' etc., which are never found in nature, but form the most convenient abstractions for measuring things. Both 'space' and 'time,' as defined for mathematical purposes, are ideal constructions drawn from empirical 'space' (extension) and 'time' (succession) feelings, and purged of the subjective variations of these experiences. Nevertheless, geometry forms the handiest system for applying to experience and calculating shapes and motions. But, ideally, other systems might be used. The 'metageometries' have constructed other ideal 'spaces' out of postulates differing from Euclid's, though when applied to real space their greater complexity destroys their value. The postulatory character of the arithmetical unit is quite as clear; for, in application, we always have to agree as to what is to count as 'one'; if we agree to count apples, and count the two halves of an apple as each equalling one, we are said to be 'wrong,' though, if we were dividing the apple among two applicants, it would be quite right to treat each half as 'one' share. Again, though one penny added to another makes two, one drop of water added to another makes one, or a dozen, according as it is dropped. Common sense, therefore, admits that we may reckon variously, and that arithmetic does not apply to all things.

Again, it is impossible to concede any meaning even to the central 'law of thought' itself—the Law of Identity ('A is A')—except as a postulate. Outside of Formal Logic and lunatic asylums no one wishes to assert that 'A is A.' All significant assertion takes the form 'A is B.' But A and B are different, and, indeed, no two 'A's' are ever quite the same. Hence, when we assert either the 'identity' of 'A' in two contexts, or that of 'A' and 'B,' in 'A is B,' we are clearly ignoring differences which really exist—i.e., we postulate that in spite of these differences A and B will for our purposes behave as if they were one ('identical'). And we should realize that this postulate is of our making, and involves a risk. It may be that experience refuses to confirm it, and convicts us instead of a 'mistaken identity.' In short, every identity we reason from is made by our postulating an irrelevance of differences.

There is thus, perhaps, no fundamental procedure of thought in which we cannot trace some deliberately adopted attitude. We distinguish between 'ourselves' and the 'external' world, perhaps because we have more control over our thoughts and limbs, and less, or none, over sticks and stones and mountains; fundamental as it is, it is a distinction within experience, and is not given ready-made, but elaborated in the course of our dealings with it. Similarly, in accordance with its varying degrees of vividness, continuity, and value, experience itself gets sorted into 'realities,' 'dreams,' and 'hallucinations.' In short, when the processes of discriminating between 'dreams' and 'reality' are considered, all these distinctions will ultimately be found to be judgments of value.

Nor is it only in the realm of scientific knowing that postulation reveals itself as a practicable and successful method of anticipating experience and consolidating fact. The same method has always been employed by man in reaching out towards the final syntheses which (in imagination) complete his vision of reality. The 'truths' of all religions originate in postulates. 'Gods' and 'devils,' 'heavens' and 'hells,' are essentially demands for a moral order in experience which transcend the given. The value of the actual world is supplemented and enhanced by being conceived as projected and continued into a greater, and our postulates are verified by the salutary influence they exercise on our earthly life. Both postulation and verification, then, are applicable to the problems of religion as of science. This is the meaning of the Will to Believe. When James first defined and defended it, it provoked abundant protest, on the ground that it allowed everyone to believe whatever he pleased and to call it 'true.' The critics had simply failed to see that verification by experience is just as integral a part of voluntaristic procedure as experimental postulation, and that James himself had from the first asserted this. Indeed, that he had first given a theological illustration of the function of volition in knowing was merely an accident. But that the will to believe was capable of being generalized into a voluntarist theory of all knowledge was soon shown in Dr. Schiller's Axioms as Postulates.