The Complete Aristotle (eng.)

necessary truths and
definition is a scientific process, and if, just as knowledge
cannot be sometimes knowledge and sometimes ignorance, but the
state which varies thus is opinion, so too demonstration and
definition cannot vary thus, but it is opinion that deals with that
which can be otherwise than as it is, clearly there can neither be
definition of nor demonstration about sensible individuals. For
perishing things are obscure to those who have the relevant
knowledge, when they have passed from our perception; and though
the formulae remain in the soul unchanged, there will no longer be
either definition or demonstration. And so when one of the
definition-mongers defines any individual, he must recognize that
his definition may always be overthrown; for it is not possible to
define such things.
    Nor is it possible to define any Idea. For the Idea is, as its
supporters say, an individual, and can exist apart; and the formula
must consist of words; and he who defines must not invent a word
(for it would be unknown), but the established words are common to
all the members of a class; these then must apply to something
besides the thing defined; e.g. if one were defining you, he would
say ‘an animal which is lean’ or ‘pale’, or something else which
will apply also to some one other than you. If any one were to say
that perhaps all the attributes taken apart may belong to many
subjects, but together they belong only to this one, we must reply
first that they belong also to both the elements; e.g. ‘two-footed
animal’ belongs to animal and to the two-footed. (And in the case
of eternal entities this is even necessary, since the elements are
prior to and parts of the compound; nay more, they can also exist
apart, if ‘man’ can exist apart. For either neither or both can.
If, then, neither can, the genus will not exist apart from the
various species; but if it does, the differentia will also.)
Secondly, we must reply that ‘animal’ and ‘two-footed’ are prior in
being to ‘two-footed animal’; and things which are prior to others
are not destroyed when the others are.
    Again, if the Ideas consist of Ideas (as they must, since
elements are simpler than the compound), it will be further
necessary that the elements also of which the Idea consists, e.g.
‘animal’ and ‘two-footed’, should be predicated of many subjects.
If not, how will they come to be known? For there will then be an
Idea which cannot be predicated of more subjects than one. But this
is not thought possible-every Idea is thought to be capable of
being shared.
    As has been said, then, the impossibility of defining
individuals escapes notice in the case of eternal things,
especially those which are unique, like the sun or the moon. For
people err not only by adding attributes whose removal the sun
would survive, e.g. ‘going round the earth’ or ‘night-hidden’ (for
from their view it follows that if it stands still or is visible,
it will no longer be the sun; but it is strange if this is so; for
‘the sun’ means a certain substance); but also by the mention of
attributes which can belong to another subject; e.g. if another
thing with the stated attributes comes into existence, clearly it
will be a sun; the formula therefore is general. But the sun was
supposed to be an individual, like Cleon or Socrates. After all,
why does not one of the supporters of the Ideas produce a
definition of an Idea? It would become clear, if they tried, that
what has now been said is true.
<
    div id="section91" class="section" title="16">
16
    Evidently even of the things that are thought to be substances,
most are only potencies,-both the parts of animals (for none of
them exists separately; and when they are separated, then too they
exist, all of them, merely as matter) and earth and fire and air;
for none of them is a unity, but as it were a mere heap, till they
are worked up and some unity is made out of them. One might most
readily suppose the parts of living things and the parts of the
soul nearly related to them to turn out to be both, i.e. existent
in complete reality as well as in potency, because they have
sources of movement in something in their joints; for which reason
some animals live when divided. Yet all the parts must exist only
potentially, when they are one and continuous by nature,-not by
force or by growing into one, for such a phenomenon is an
abnormality.
    Since the term ‘unity’ is used like the term ‘being’, and