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