Hence the rule of simplicity
is in favour of day and night being caused by the revolving of the earth
on its axis. The same rule might be illustrated in many ways; but,
however illustrated, the principle, according to Newton, always holds
good that all effects are produced by the simplest causes, and if there
are apparently two causes to the same phenomenon, then the simpler cause
is the true and correct one. So that in the making and development of
any hypotheses of the physical cause of Gravitation, this rule of
simplicity must always be recognized; and, in conjunction with the other
rules, we must seek to make our hypotheses, so as to be able to account
and explain all phenomena sought to be explained.

ART. 5. _2nd Rule. Experience._--Newton fully recognized the necessity
of experience in Philosophy. He saw the absolute necessity of appealing
to experience, observation, and experiment, both as a basis for
philosophical reasoning, and further, for the data which were necessary
to verify particular applications of the hypotheses suggested.

In his Rules of Philosophy, referring to experience as a guide, he says:
"Hoc est fundamentum philosophiae."--"This is the basis of philosophy."

Herschel, writing on the same subject in his _Natural Philosophy_,
writes thus with regard to experience: "We have pointed out that the
great, and indeed the only ultimate source of our knowledge of nature,
and its laws, is experience. By which I mean, not the experience of one
man only, or of one generation, but the accumulated experience of all
mankind in all ages registered in books or recorded in tradition. But
experience may be acquired in two ways, either first by noticing facts
as they occur without any attempt to influence the frequency of their
occurrence, or to vary the circumstances under which they occur. This is
observation. Second, by putting in action causes and agents over which
we have no control, and purposely varying their combination, and then
noticing what effects take place. This is experiment. To these two
sources we must look as the fountains of all natural science."

Herschel further writes: "Experience once recognized as the fountain of
all our knowledge of nature, it follows, that in our study of nature and
its laws, we ought at once to make up our minds to dismiss, as idle
prejudices, or at least suspend as premature, all preconceived notion of
what might, or ought to be the order of nature in any proposed