on, or even any one century, would have seen the complete
substitution of exact for inexact ideas. Slowness of growth is as
inevitable in the case of knowledge as in that of a growing organism.
The most essential point of difference is one of those seemingly slight
ones, the importance of which we are too apt to overlook. It was like
the drop of blood in the wrong place, which some one has told us makes
all the difference between a philosopher and a maniac. It was all the
difference between a living tree and a dead one, between an inert mass
and a growing organism. The transition of knowledge from the dead to
the living form must, in any complete review of the subject, be looked
upon as the really great event of modern times. Before this event the
intellect was bound down by a scholasticism which regarded knowledge as
a rounded whole, the parts of which were written in books and carried
in the minds of learned men. The student was taught from the beginning
of his work to look upon authority as the foundation of his beliefs.
The older the authority the greater the weight it carried. So effective
was this teaching that it seems never to have occurred to individual
men that they had all the opportunities ever enjoyed by Aristotle of
discovering truth, with the added advantage of all his knowledge to
begin with. Advanced as was the development of formal logic, that
practical logic was wanting which could see that the last of a series
of authorities, every one of which rested on those which preceded it,
could never form a surer foundation for any doctrine than that supplied
by its original propounder.

The result of this view of knowledge was that, although during the
fifteen centuries following the death of the geometer of Syracuse great
universities were founded at which generations of professors expounded
all the learning of their time, neither professor nor student ever
suspected what latent possibilities of good were concealed in the most
familiar operations of Nature. Every one felt the wind blow, saw water
boil, and heard the thunder crash, but never thought of investigating
the forces here at play. Up to the middle of the fifteenth century the
most acute observer could scarcely have seen the dawn of a new era.

In view of this state of things it must be regarded as one of the most
remarkable facts in evolutionary history that four or five men, whose
mental constitution was either typical of the new order of things, o