stence simultaneously. This is
for the most part true, as in the case of the double and the half. The
existence of the half necessitates the existence of that of which it is
a half. Similarly the existence of a master necessitates the existence
of a slave, and that of a slave implies that of a master; these are
merely instances of a general rule. Moreover, they cancel one another;
for if there is no double it follows that there is no half, and vice
versa; this rule also applies to all such correlatives. Yet it does not
appear to be true in all cases that correlatives come into existence
simultaneously. The object of knowledge would appear to exist before
knowledge itself, for it is usually the case that we acquire knowledge
of objects already existing; it would be difficult, if not impossible,
to find a branch of knowledge the beginning of the existence of which
was contemporaneous with that of its object.

Again, while the object of knowledge, if it ceases to exist, cancels at
the same time the knowledge which was its correlative, the converse of
this is not true. It is true that if the object of knowledge does not
exist there can be no knowledge: for there will no longer be anything
to know. Yet it is equally true that, if knowledge of a certain object
does not exist, the object may nevertheless quite well exist. Thus, in
the case of the squaring of the circle, if indeed that process is an
object of knowledge, though it itself exists as an object of knowledge,
yet the knowledge of it has not yet come into existence. Again, if all
animals ceased to exist, there would be no knowledge, but there might
yet be many objects of knowledge.

This is likewise the case with regard to perception: for the object of
perception is, it appears, prior to the act of perception. If the
perceptible is annihilated, perception also will cease to exist; but
the annihilation of perception does not cancel the existence of the
perceptible. For perception implies a body perceived and a body in
which perception takes place. Now if that which is perceptible is
annihilated, it follows that the body is annihilated, for the body is a
perceptible thing; and if the body does not exist, it follows that
perception also ceases to exist. Thus the annihilation of the
perceptible involves that of perception.

But the annihilation of perception does not involve that of the
perceptible. For if the animal is annihilated, it follows that
perception also is a