The Complete Aristotle (eng.)

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 anything, and that knowledge of essential
nature is not to be obtained either by definition or by
demonstration.
8
    We must now start afresh and consider which of these conclusions
are sound and which are not, and what is the nature of definition,
and whether essential nature is in any sense demonstrable and
definable or in none.
    Now to know its essential nature is, as we said, the same as to
know the cause of a thing’s existence, and the proof of this
depends on the fact that a thing must have a cause. Moreover, this
cause is either identical with the essential nature of the thing or
distinct from it; and if its cause is distinct from it, the
essential nature of the thing is either demonstrable