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Works by Aristotle
Pages of Metaphysics

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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
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.

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

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