ree following heads--

(1) Simplicity of conception.

(2) Agreement with experience, observation, and experiment.

(3) Satisfactorily accounting for, and explaining all phenomena
sought to be explained.

ART. 4. _1st Rule. Simplicity of Conception._--From this rule we learn
that the hypothesis must be simple in conception, and simple in its
fundamental principles, and further, that the same characteristic of
simplicity must mark each step of its development.

This rule of simplicity is distinctly laid down by Sir Isaac Newton in
his _Principia_, Book 3, under the heading "Regulae Philosophandi."

In that work he writes: "Natura simplex est, et rerum causis superfluis
non luxuriat."--"Nature is simple, and does not abound in superfluous
causes of things."

He further states that: "Not more of the natural causes of things ought
to be admitted, than those which are true and suffice to explain
phenomena. In the nature of Philosophy nothing is done in vain, and by
means of many things, it is done in vain when it can be done by fewer.
For Nature is simple, and does not abound in superfluous causes."

While again in Rule 3, he adds: "Natura simplex est et sibi semper
consona."--"Nature is simple, and always agrees with itself."

Whewell also considers simplicity as a fundamental principle of all true
hypotheses. On this point he writes: "All the hypotheses should tend to
simplicity and harmony. The new suppositions resolve themselves into the
old ones, or at least only require some easy modification of the
hypothesis first assumed. In false theories the contrary is the case."

Thus, it is the very essence of philosophy to build upon a foundation of
simplicity, combined with the results of experience, observation, and
experiment. For example, if we desired to form a hypothesis as to the
cause of day and night, two hypotheses might be assigned as to the
cause.

First, that the earth revolves on its axis once a day, and so presents
each part successively to the light and heat of the sun; and second,
that the sun revolves round the earth once every 24 hours. But such an
assumption as the latter would involve the revolution of the sun through
an immense orbit at an enormous velocity, in order for the journey to be
accomplished in the time. So that it is much simpler to conceive of the
earth revolving on its axis once every 24 hours, than it is for the sun
to perform this journey in the same period.