ets the second point, that we call extension by itself
_space_, and not body, by maintaining that the distinction between
extension and corporeal substance is a distinction in thought, and not in
reality; that attribute and substance, mathematical and physical bodies,
are not distinct in fact but only in our thought of them. We apply the
term space to extension in general, as an abstraction, and body to a given
individual, determinate, limited extension. In reality, wherever extension
is, there substance is also,--the non-existent has no extension,--and
wherever space is, there matter is also. Empty space does not exist.
When we say a vessel is empty, we mean that the bodies which fill it are
imperceptible; if it were absolutely empty its sides would touch. Descartes
argues against the atomic theory and against the finitude of the world, as
he argues against empty space: matter, as well as space, has no smallest,
indivisible parts, and the extension of the world has no end. In the
identification of space and matter the former receives fullness from
the latter, and the latter unlimitedness from the former, both internal
unlimitedness (endless divisibility) and external (boundlessness). Hence
there are not several matters but only one (homogeneous) matter, and only
one (illimitable) world.

[Footnote 1: They are merely subjective states in the perceiver, and
entirely unlike the motions which give rise to them, although there is
a certain agreement, as the differences and variations in sensation are
paralleled by those in the object.]

Matter is divisible, figurable, movable quantity. Natural science needs no
other principles than these indisputably true conceptions, by which all
natural phenomena may be explained, and must employ no others. The most
important is motion, on which all the diversity of forms depends. Corporeal
being has been shown to be extension; corporeal becoming is motion. Motion
is defined as "the transporting of one part of matter, or of one body, from
the vicinity of those bodies that are in immediate contact with it,
or which we regard as at rest, to the vicinity of other bodies." This
separation of bodies is reciprocal, hence it is a matter of choice which
shall be considered at rest. Besides its own proper motion in reference to
the bodies in its immediate vicinity, a body can participate in very many
other motions: the traveler walking back and forth on the deck of a ship,
for instance, in the mot