or cause of Gravitation, therefore,
among the phenomena and laws, which have to be satisfactorily accounted
for on a physical basis, are these three Laws of Kepler's just referred
to.

So that in addition to the satisfactory explanation of a physical cause
for the Laws of Motion, and the Centripetal and Centrifugal Forces, the
hypothesis of a physical cause of Gravitation must fully and
satisfactorily account for the Laws of Kepler, whose mathematical
explanation was given by Newton.

Further, and what is as equally important, the explanation so given must
be strictly in harmony with the Rules of Philosophy as laid down in Art.
3. That is, the explanation must be simple in character, must not be
contrary to experience or observation, and must satisfactorily account
for the laws which the hypothesis of the physical cause of Gravitation
seeks to explain. This I premise we will do as we pass from stage to
stage in the development of the theory.

I can safely premise that it will be simple in character and conception,
that it will be entirely in harmony with all experience and observation,
and that the physical cause so advanced will give as physical a basis to
Kepler's Laws as Newton's mathematical calculations gave them a
mathematical basis.

In summing up, I need hardly point out, that if all that I have premised
in this and the preceding chapter is accomplished in the after chapters
of this book, then for the first time since the discovery of Universal
Gravitation by Sir Isaac Newton, his great discovery will have received
the long-expected and long-desired physical explanation, that
explanation and cause being founded on his own Rules of Philosophy given
in his immortal _Principia_, and for the first time our Philosophy will
be brought strictly into harmony with our universal experience.

CHAPTER III

MATTER

ART. 29. _What is Matter?_--The law of Universal Attraction states that
"Every particle of matter attracts every other particle," etc., and the
question at once arises as to what is meant by the term Matter, what are
its properties and its constitution? Tait, in his _Natural Philosophy_,
gives the following as the definition: "Matter is that which can be
perceived by the senses, or is that which can be acted upon by, or can
exert force."

It has already been pointed out in Art. 13 that force is due to motion,
and that wherever we get