Posterior Analytics   

'that'; but in this premiss the term we assert of the minor is neither
the major itself nor a term identical in definition, or convertible,
with the major.
Again, both proof by division and the syllogism just described are
open to the question why man should be animal-biped-terrestrial and
not merely animal and terrestrial, since what they premise does not
ensure that the predicates shall constitute a genuine unity and not
merely belong to a single subject as do musical and grammatical when
predicated of the same man.

7

How then by definition shall we prove substance or essential nature?
We cannot show it as a fresh fact necessarily following from the
assumption of premisses admitted to be facts-the method of
demonstration: we may not proceed as by induction to establish a
universal on the evidence of groups of particulars which offer no
exception, because induction proves not what the essential nature of a
thing is but that it has or has not some attribute. Therefore, since
presumably one cannot prove essential nature by an appeal to sense
perception or by pointing with the finger, what other method remains?
To put it another way: how shall we by definition prove essential
nature? He who knows what human-or any other-nature is, must know also
that man exists; for no one knows the nature of what does not
exist-one can know the meaning of the phrase or name 'goat-stag' but
not what the essential nature of a goat-stag is. But further, if
definition can prove what is the essential nature of a thing, can it
also prove that it exists? And how will it prove them both by the same
process, since definition exhibits one single thing and
demonstration another single thing, and what human nature is and the
fact that man exists are not the same thing? Then too we hold that
it is by demonstration that the being of everything must be
proved-unless indeed to be were its essence; and, since being is not a
genus, it is not the essence of anything. Hence the being of
anything as fact is matter for demonstration; and this is the actual
procedure of the sciences, for the geometer assumes the meaning of the
word triangle, but that it is possessed of some attribute he proves.
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. It appears then from these considerations that
neither definition and syllogism nor their objects are identical,
and further that definition neither demonstrates nor proves