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 scientific work is aware that those who refuse to go beyond fact, rarely get as far as fact; and anyone who has studied the history of science knows that almost every great step therein has been made by the ‘anticipation of Nature,’ that is, by the invention of hypotheses, which, though verifiable, often had very little foundation to start with; and, not unfrequently, in spite of a long career of usefulness, turned out to be wholly erroneous in the long run.

[Sidenote:  Fruitful use of an hypothesis even when wrong.]

The geocentric system of astronomy, with its eccentrics and its epicycles, was an hypothesis utterly at variance with fact, which nevertheless did great things for the advancement of astronomical knowledge.  Kepler was the wildest of guessers.  Newton’s corpuscular theory of light was of much temporary use in optics, though nobody now believes in it; and the undulatory theory, which has superseded the corpuscular theory and has proved one of the most fertile of instruments of research, is based on the hypothesis of the existence of an ‘ether,’ the properties of which are defined in propositions, some of which, to ordinary apprehension, seem physical antinomies.