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believes in it; and the undulatory theory, which has superseded the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.

It sounds paradoxical to say that the attainment of scientific truth
has been effected, to a great extent, by the help of scientific
errors. But the s