What is it, then, that we shall prove in defining essential nature?
Triangle? In that case a man will know by definition what a thing's
nature is without knowing whether it exists. But that is impossible.
Moreover it is clear, if we consider the methods of defining
actually in use, that definition does not prove that the thing defined
exists: since even if there does actually exist something which is
equidistant from a centre, yet why should the thing named in the
definition exist? Why, in other words, should this be the formula
defining circle? One might equally well call it the definition of
mountain copper. For definitions do not carry a further guarantee that
the thing defined can exist or that it is what they claim to define:
one can always ask why.
Since, therefore, to define is to prove either a thing's essential
nature or the meaning of its name, we may conclude that definition, if
it in no sense proves essential nature, is a set of words signifying
precisely what a name signifies. But that were a strange
consequence; for (1) both what is not substance and what does not
exist at all would be definable, since even non-existents can be
signified by a name: (2) all sets of words or sentences would be
definitions, since any kind of sentence could be given a name; so that
we should all be talking in definitions, and even the Iliad would be a
definition: (3) no demonstration can prove that any particular name
means any particular thing: neither, therefore, do definitions, in
addition to revealing the meaning of a name, also reveal that the name
has this meaning.
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