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.
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