of the natural forces or normal
categories of physical action that Galileo and Newton devoted
themselves. The true estimate of a moving force may indeed be said to
have been their first great problem, just as the law of universal
gravitation was their grandest generalisation.

It was to this sure instinct that the founders of Science owed their
success. Had they devoted themselves to the mere study of sensations--of
blue things and green things, of hard things and soft things, of loud
things and silent things--Science as an efficient and co-ordinated
system would never have come into being.

* * * * *

Having struck the right path, they moved rapidly along it, leaving the
Schoolmen and Philosophers behind them, suspicious, hostile, and amazed.

But Philosophy did not remain altogether negative. The new movement
extended itself to Metaphysics, and under the leadership of Descartes a
resolute effort was made to reform Philosophy on sympathetic lines.

It was in the true spirit of Socrates that Descartes advanced his famous
method of Doubt. The whole fabric of beliefs and rational principles was
to be subjected to a re-examination, and Descartes found himself on
bedrock when he touched his famous _Cogito, ergo sum._ The simple fact
or act of Doubt implied the Activity--the Reality therefore--of the
Doubter. But the cogitant subject was reduced very much to the condition
of a _tabula rasa_, and when Descartes proceeded to fill up the blank
with a rediscovery on more scientific lines of the essentials of
Cognition he found his basal feature in Extension. Tridimensional Space
seemed the simple elementary framework of our Knowledge of Nature.

The method of Descartes was further extended by the English philosopher
Locke. Those qualities which formed the elements of Knowledge were
described by him as the primary qualities of body; the sensible
presentation comprised also the secondary qualities which seemed to be
in some way superposed upon and contained within the former.

* * * * *

Our fundamental ideas of Nature were called by Locke sensible ideas.
These ideas were derived from our sensible Experience, and it is only
just to Locke to point out that, when examined in detail, his sensible
ideas are seen to be not mere qualifications of sensation, but rather
the elementary characters of Nature viewed as a dynamic process and
discovered by our Activity. Yet