not come from ourselves, is shown by
the fact that it is not in our power to determine the objects which we
perceive, or the character of our perception of them. The supposition that
God has caused our perceptions directly, or by means of something which has
no resemblance whatever to an external object extended in three dimensions
and movable, is excluded by the fact that God is not a deceiver. In
reliance on God's veracity we may accept as true whatever the reason
declares concerning body, though not all the reports of the senses,
which so often deceive us. At the instance of the senses we clearly and
distinctly perceive matter distinct from our mind and from God, extended
in three dimensions, length, breadth, and depth, with variously formed and
variously moving parts, which occasion in us sensations of many kinds. The
belief that perception makes known things as they really are is a prejudice
of sense to be discarded; on the contrary, it merely informs us concerning
the utility or harmfulness of objects, concerning their relation to man as
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 me