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believes in it; and the undulatory theory, which has superseded the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 subject-matter of physical science is furnished by
observation, which cannot extend beyond the limits of our faculties;
while, even within those limits, we cannot be certain that any
observation is absolutely exact and exhaustive. Hence it follows that
any given generalisation from observation may be true, within the
limits of our powers of observation at a given time, and yet turn out
to be untrue, when those powers of observation are directly or