fullness of time.

Secondly, all scientific truth is conditioned by the state of knowledge
at the time of its announcement. Thus, at the beginning of the
seventeenth century, the science of optics and mechanical appliances had
not made possible (so far as the human mind was concerned) the existence
of blood capillaries and blood corpuscles. Jenner could not have
added to his "Inquiry" a study on immunity; Sir William Perkin and the
chemists made Koch technique possible; Pasteur gave the conditions that
produced Lister; Davy and others furnished the preliminaries necessary
for anaesthesia. Everywhere we find this filiation, one event following
the other in orderly sequence--"Mind begets mind," as Harvey (De
Generatione) says; "opinion is the source of opinion. Democritus with
his atoms, and Eudoxus with his chief good which he placed in pleasure,
impregnated Epicurus; the four elements of Empedocles, Aristotle;
the doctrines of the ancient Thebans, Pythagoras and Plato; geometry,
Euclid."(2)

(2) Works of William Harvey, translated by Robert Willis, London,
1847, p. 532.

And, thirdly, to scientific truth alone may the homo mensura principle
be applied, since of all mental treasures of the race it alone compels
general acquiescence. That this general acquiescence, this aspect of
certainty, is not reached per saltum, but is of slow, often of difficult
growth,--marked by failures and frailties, but crowned at last with
an acceptance accorded to no other product of mental activity,--is
illustrated by every important discovery from Copernicus to Darwin.

The difficulty is to get men to the thinking level which compels the
application of scientific truths. Protagoras, that "mighty-wise man,"
as Socrates called him, who was responsible for the aphorism that man
is the measure of all things, would have been the first to recognize the
folly of this standard for the people at large. But we have gradually
reached a stage in which knowledge is translated into action, made
helpful for suffering humanity, just as the great discoveries in physics
and chemistry have been made useful in the advance of civilization.
We have traced medicine through a series of upward steps--a primitive
stage, in which it emerged from magic and religion into an empirical
art, as seen among the Egyptians and Babylonians; a stage in which the
natural character of disease was recognized and the importance of its
study as a phenomenon of nature