invention of verifiable hypotheses is not only permissible, but is one
of the conditions of progress.

[Sidenote: and mutual assistance of observation, experiment, and
speculation.]

Historically, no branch of science has followed this order of growth;
but, from the dawn of exact knowledge to the present day, observation,
experiment, and speculation have gone hand in hand; and, whenever
science has halted or strayed from the right path, it has been, either
because its votaries have been content with mere unverified or
unverifiable speculation (and this is the commonest case, because
observation and experiment are hard work, while speculation is
amusing); or it has been, because the accumulation of details of
observation has for a time excluded speculation.

[Sidenote: Recognition of these truths in recent times, and consequent
progress.]

The progress of physical science, since the revival of learning, is
largely due to the fact that men have gradually learned to lay aside
the consideration of unverifiable hypotheses; to guide observation
and experiment by verifiable hypotheses; and to consider the latter,
not as ideal truths, the real entities of an intelligible world behind
phenomena, but as a symbolical language, by the aid of which nature
can be interpreted in terms apprehensible by our intellects. And if
physical science, during the last fifty years, has attained dimensions
beyond all former precedent, and can exhibit achievements of greater
importance than any former such period can show, it is because able
men, animated by the true scientific spirit, carefully trained in the