iving 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 'irrelev