relative motion is that which is perceived by sense, and regarded in
the ordinary affairs of life, it should seem that every man of common
sense knows what it is as well as the best philosopher. Now, I ask any
one whether, in his sense of motion as he walks along the streets, the
stones he passes over may be said to move, because they change distance
with his feet? To me it appears that though motion includes a relation of
one thing to another, yet it is not necessary that each term of the
relation be denominated from it. As a man may think of somewhat which
does not think, so a body may be moved to or from another body which is
not therefore itself in motion.

114. As the place happens to be variously defined, the motion which is
related to it varies. A man in a ship may be said to be quiescent with
relation to the sides of the vessel, and yet move with relation to the
land. Or he may move eastward in respect of the one, and westward in
respect of the other. In the common affairs of life men never go beyond
the earth to define the place of any body; and what is quiescent in
respect of that is accounted absolutely to be so. But philosophers, who
have a greater extent of thought, and juster notions of the system of
things, discover even the earth itself to be moved. In order therefore to
fix their notions they seem to conceive the corporeal world as finite,
and the utmost unmoved walls or shell thereof to be the place whereby
they estimate true motions. If we sound our own conceptions, I believe we
may find all the absolute motion we can frame an idea of to be at bottom
no other than relative motion thus defined. For, as has been already
observed, absolute motion, exclusive of all external relation, is
incomprehensible; and to this kind of relative motion all the
above-mentioned properties, causes, and effects ascribed to absolute
motion will, if I mistake not, be found to agree. As to what is said of
the centrifugal force, that it does not at all belong to circular