[Sidenote: It is based on postulates]

All physical science starts from certain postulates. One of them is
the objective existence of a material world. It is assumed that the
phenomena which are comprehended under this name have a 'substratum'
of extended, impenetrable, mobile substance, which exhibits the
quality known as inertia, and is termed matter.[E] Another postulate
is the universality of the law of causation; that nothing happens
without a cause (that is, a necessary precedent condition), and that
the state of the physical universe, at any given moment, is the
consequence of its state at any preceding moment. Another is that any
of the rules, or so-called 'laws of nature,' by which the relation of
phenomena is truly defined, is true for all time. The validity of
these postulates is a problem of metaphysics; they are neither
self-evident nor are they, strictly speaking, demonstrable. The
justification of their employment, as axioms of physical philosophy,
lies in the circumstance that expectations logically based upon them
are verified, or, at any rate, not contradicted, whenever they can be
tested by experience.

[Sidenote: and uses hypotheses.]

Physical science therefore rests on verified or uncontradicted
hypotheses; and, such being the case, it is not surprising that a
great condition of its progress has been the invention of verifiable
hypotheses. It is a favorite popular delusion that the scientific
inquirer is under a sort of moral obligation to abstain from going
beyond that generalisation of observed facts which is absurdly called
'Baconian' induction. But anyone who is practically acquainted with