a being composed of soul and body. (The body is that material thing which
is very intimately joined with the mind, and occasions in the latter
certain feelings, _e.g._, pain, which as merely cogitative it would not
have.) Sense qualities, as color, sound, odor, cannot constitute the
essence of matter, for their variation or loss changes nothing in it; I can
abstract from them without the material thing disappearing.[1] There is one
property, however, extensive magnitude (_quantitas_), whose removal would
imply the destruction of matter itself. Thus I perceive by pure thought
that the essence of matter consists in extension, in that which constitutes
the object of geometry, in that magnitude which is divisible, figurable,
and movable. This thesis (_corpus = extensio sive spatium_) is next
defended by Descartes against several objections. In reply to the objection
drawn from the condensation and rarefaction of bodies, he urges that the
apparent increase or decrease in extension is, in fact, a mere change of
figure; that the rarefaction of a body depends on the increase in size of
the intervals between its parts, and the entrance into them of foreign
bodies, just as a sponge swells up when its pores become filled with water
and, therefore, enlarged. The demand that the pores, and the bodies which
force their way into them, should always be perceptible to the senses, is
groundless. He meets 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