The Complete Aristotle (eng.)

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 annihilated, but perceptibles such as body,
heat, sweetness, bitterness, and so on, will remain.
    Again, perception is generated at the same time as the
perceiving subject, for it comes into existence at the same time as
the animal. But the perceptible surely exists before perception;
for fire and water and such elements, out of which the animal is
itself composed, exist before the animal is an animal at all, and
before perception. Thus it would seem that the perceptible exists
before perception.
    It may be questioned whether it is true that no substance is
relative, as seems to be the case, or whether exception is to be
made in the case of certain secondary substances. With regard to
primary substances, it is quite true that there is no such
possibility, for neither wholes nor parts of primary substances are
relative. The individual man or ox is not defined with reference to
something external. Similarly with the parts: a particular hand or
head is not defined as a particular hand or head of a particular
person, but as the hand or head of a particular person. It is true
also, for the most part at least, in the case of secondary
substances; the species ‘man’ and the species ‘ox’ are not defined
with reference to anything outside themselves. Wood, again, is only
relative in so far as it is some one’s property, not in so far as
it is wood. It is plain, then, that in the cases mentioned
substance is not relative. But with regard to some secondary
substances there is a difference of opinion; thus, such terms as
‘head’ and ‘hand’ are defined with reference to that of which the
things indicated are a part, and so it comes about that these
appear to have a relative character. Indeed, if our definition of
that which is relative was complete, it is very difficult, if not
impossible, to prove that no substance is relative. If, however,
our definition was not complete, if those things only are properly
called relative in the case of which relation to an external object
is a necessary condition of existence, perhaps some explanation of
the dilemma may be found.
    The former definition does indeed apply to all relatives, but
the fact that a thing is explained with reference to something else
does not make it essentially relative.
    From this it is plain that, if a man definitely apprehends a
relative thing, he will also definitely apprehend that to which it
is relative. Indeed this is self-evident: for if a man knows that
some particular thing is relative, assuming that we call that a
relative in the case of which relation to something is a necessary
condition of existence, he knows that also to which it is related.
For if he does not know at all that to which it is related, he will
not know whether or not it is relative. This is clear, moreover, in
particular instances. If a man knows definitely that such and such
a thing is ‘double’, he will also forthwith know definitely that of
which it is the double. For if there is nothing definite of which
he knows it to be the double, he does not know at all that it is
double. Again, if he knows that a thing is more beautiful, it
follows necessarily that he will forthwith definitely know that
also than which it is more beautiful. He will not merely know
indefinitely that it is more beautiful than something which is less
beautiful, for this would be supposition, not knowledge. For if he
does not know definitely that than which it is more beautiful, he
can no longer claim to know definitely that it is more beautiful
than something else which is less beautiful: for it might be that
nothing was less beautiful. It is, therefore, evident that if a man
apprehends some relative thing definitely, he necessarily knows
that also definitely to which it is related.
    Now the head, the hand, and such things are substances, and it
is possible to know their essential character definitely, but it
does not necessarily follow that we should know that to which they
are related. It is not possible to know forthwith