It might, I mean, be urged that definition is held to
concern essential nature and is in every case universal and
affirmative; whereas, on the other hand, some conclusions are negative
and some are not universal; e.g. all in the second figure are
negative, none in the third are universal. And again, not even all
affirmative conclusions in the first figure are definable, e.g. 'every
triangle has its angles equal to two right angles'. An argument
proving this difference between demonstration and definition is that
to have scientific knowledge of the demonstrable is identical with
possessing a demonstration of it: hence if demonstration of such
conclusions as these is possible, there clearly cannot also be
definition of them. If there could, one might know such a conclusion
also in virtue of its definition without possessing the
demonstration of it; for there is nothing to stop our having the one
without the other.
Induction too will sufficiently convince us of this difference;
for never yet by defining anything-essential attribute or accident-did
we get knowledge of it. Again, if to define is to acquire knowledge of
a substance, at any rate such attributes are not substances.
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