Since an early date, this important fundamental concept has frequently
been used in three different and divergent senses, namely:
_A._ In its widest sense, as synonymous with _monism_; wherein mechanical
causes (_causæ efficientes_), in the sense of Kant, are assumed as the
sole effective causes and are placed in opposition to the teleological
causes (_causæ finales_) in the sense of dualism. “Mechanical conception
of the world” is in this sense synonymous with “monistic conception of
the world.”
_B._ In its more restricted sense, as a universal _motion_-principle
of physics, so that, for example, the postulated ether-vibrations of
optics, of electricity, and so forth, as well as the grosser material
oscillations of acoustics, heat, and so forth, are designated as
mechanical processes subject to definite laws. “Mechanical natural
philosophy,” in this sense, is identical with _physics_.
_C._ In its narrowest sense, as that _branch_ of physics which deals
with the grosser and visible processes of _motion_; as gravitation,
locomotion, and the phoronomy of organisms. Mechanics, in this the most
restricted sense, is viewed as opposed to optics, acoustics, etc., as the
usages of the schools indicate.
Since, now, the phrases “mechanical laws” and “mechanical
explanation,” at the present day even, are frequently understood in
these three distinct senses, no end of misunderstandings arise. Such
misunderstandings may be best avoided, perhaps, by retaining the notion
of mechanics in its narrowest (_C_) sense, and by substituting _physics_
for the next narrower sense (_B_) and _monism_ for its most extended
sense (_A_).