Elements of the philosophy of the human mind. By Dugald Stewart. Rev. and abridged, with critical and explanatory notes, for the use of colleges and schools. By Francis Bowen ...

REASONING AND DEDUCTIVE EVIDENCE. 443
2. Another circumstance which has cooperated powerfully
with the former in producing the same effect, is that proneness
to simplification which has misled the mind, more or less, in all
its researches, and which, in natural philosophy, is peculiarly
encouraged by those beautiful analogies which are observable
among different physical phenomena, - analogies, at the same
time, which, however pleasing to the fancy, cannot always be
resolved by our reason into one general law. In a remarkable
analogy, for example, which presents itself between the equality
of action and reaction in the collision of bodies, and what obtains in their mutual attractions, the coincidence is so perfect,
as to enable us to comprehend all the various facts in the same
theorem; and it is difficult to resist the temptation which it
seems to offer to our ingenuity, of attempting to trace it, in both
cases, to some common principle. Such trials of theoretical
skill, I would not be understood to censure indiscriminately;
but in the present instance, I am fully persuaded, that it is at
once more unexceptionable in point of sound logic, and more
satisfactory to the learner, to establish the fact, in particular
cases, by an appeal to experiment; and to state the law of
action and reaction in the collision of bodies, as well as that
which regulates the mutual tendencies of bodies towards each
other, merely as general rules which have been obtained by
induction, and rwhich are found to hold invariably as far as our
knowledge of nature extends.
3. To these remarks it may be added, that even when one
proposition in natural philosophy is logically deducible firom
another, it may frequently be expedient, in communicating the
elements of the science, to illustrate and confirm the consequence,
as well as the principle, by experiment. This I should apprehiend to be proper wherever a consequence is inferred firom a
principle less familiar and intelligible than itself; a thing which
murst occasionally happen in physics, from the complete incorporation, if I may use the expression, which, in modern times,
has taken place between physical truths, and the discoveries of
mathematicians. The necessary efiect of this incorporation was,
to give to natural philosophy a mathematical form, and to sys