t has done nothing but prove that
things must be exactly as they had been found to be by the observer and
collector. Physical science, however, would never have been what it is
without the impulses which it received from the philosopher, nay even from
the poet. "At the limits of exact knowledge" (I quote the words of
Humboldt), "as from a lofty island-shore, the eye loves to glance towards
distant regions. The images which it sees may be illusive; but, like the
illusive images which people imagined they had seen from the Canaries or
the Azores, long before the time of Columbus, they may lead to the
discovery of a new world."

Copernicus, in the dedication of his work to Pope Paul III. (it was
commenced in 1517, finished 1530, published 1543), confesses that he was
brought to the discovery of the sun's central position, and of the diurnal
motion of the earth, not by observation or analysis, but by what he calls
the feeling of a want of symmetry in the Ptolemaic system. But who had
told him that there _must_ be symmetry in all the movements of the
celestial bodies, or that complication was not more sublime than
simplicity? Symmetry and simplicity, before they were discovered by the
observer, were postulated by the philosopher. The first idea of
revolutionizing the heavens was suggested to Copernicus, as he tells us
himself, by an ancient Greek philosopher, by Philolaus, the Pythagorean.
No doubt with Philolaus the motion of the earth was only a guess, or, if
you like, a happy intuition. Nevertheless, if we may trust the words of
Copernicus, it is quite possible that without that guess we should never
have heard of the Copernican system. Truth is not found by addition and
multiplication only. When speaking of Kepler, whose method of reasoning
has been considered as unsafe and fantastic by his contemporaries as well
as by later astronomers, Sir David Brewster remarks very truly, "that, as
an instrument of research, the influence of imagination has been much
overlooked by those who have ventured to give laws to philosophy." The
torch of imagination is as necessary to him who looks for truth, as the
lamp of study. Kepler held both, and more than that, he had the star of
faith to guide him in all things from darkness to light.

In the history of the physical sciences, the three stages which we have
just described as the empirical, the classificatory, and the theoretical,
appear generally in chronological order. I say, generall