The Advance of Science in the Last Half-Century | Page 7

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 indirectly enlarged. Or, to put the
matter in another way, a doctrine which is untrue absolutely, may, to a
very great extent, be susceptible of an interpretation in accordance with
the truth. At a certain period in the history of astronomical science, the
assumption that the planets move in circles was true enough to serve
the purpose of correlating such observations as were then possible;
after Kepler, the assumption that they move in ellipses became true
enough in regard to the state of observational astronomy at that time.
We say still that the orbits of the planets are ellipses, because, for all
ordinary purposes, that is a sufficiently near approximation to the truth;
but, as a matter of fact, the centre of gravity of a planet describes
neither an ellipse or any other simple curve, but an immensely
complicated undulating line. It may fairly be doubted whether any
generalisation, or hypothesis, based upon physical data is absolutely
true, in the sense that a mathematical proposition is so; but, if its errors
can become apparent only outside the limits of practicable observation,
it may be just as usefully adopted for one of the symbols of that algebra
by which we interpret nature, as if it were absolutely true.
The development of every branch of physical knowledge presents three
stages which, in their logical relation, are successive. The first is the
determination of the sensible character and order of the phenomena.
This is Natural History, in the original sense of the term, and here
nothing but observation and experiment avail us. The second is the
determination of the constant relations of the phenomena thus defined,
and their expression in rules or laws. The third is the explication of
these particular laws by deduction from the most general laws of matter

and motion. The last two stages constitute Natural Philosophy in its
original sense. In this region, the 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 method of science,
and having at their disposal immensely improved appliances, have
devoted themselves to the enlargement of the boundaries of natural
knowledge in greater number than during any previous half-century of
the world's history.
[Sidenote: The three great achievements. Doctrines of (1) molecular
constitution of matter, (2) conservation of energy, (3) evolution.]

I have said that our epoch can produce achievements in physical
science of greater moment than any other has to show, advisedly; and I
think that there are three great products of our time which justify the
assertion. One of these is that doctrine concerning the constitution of
matter which, for want of a better name, I will call 'molecular;' the
second is the doctrine of conservation of energy; the third is the
doctrine of evolution. Each of these was foreshadowed, more or less