The Complete Aristotle (eng.)

or
indemonstrable. Consequently, if the cause is distinct from the
thing’s essential nature and demonstration is possible, the cause
must be the middle term, and, the conclusion proved being universal
and affirmative, the proof is in the first figure. So the method
just examined of proving it through another essential nature would
be one way of proving essential nature, because a conclusion
containing essential nature must be inferred through a middle which
is an essential nature just as a ‘peculiar’ property must be
inferred through a middle which is a ‘peculiar’ property; so that
of the two definable natures of a single thing this method will
prove one and not the other.
    Now it was said before that this method could not amount to
demonstration of essential nature-it is actually a dialectical
proof of it-so let us begin again and explain by what method it can
be demonstrated. When we are aware of a fact we seek its reason,
and though sometimes the fact and the reason dawn on us
simultaneously, yet we cannot apprehend the reason a moment sooner
than the fact; and clearly in just the same way we cannot apprehend
a thing’s definable form without apprehending that it exists, since
while we are ignorant whether it exists we cannot know its
essential nature. Moreover we are aware whether a thing exists or
not sometimes through apprehending an element in its character, and
sometimes accidentally, as, for example, when we are aware of
thunder as a noise in the clouds, of eclipse as a privation of
light, or of man as some species of animal, or of the soul as a
self-moving thing. As often as we have accidental knowledge that
the thing exists, we must be in a wholly negative state as regards
awareness of its essential nature; for we have not got genuine
knowledge even of its existence, and to search for a thing’s
essential nature when we are unaware that it exists is to search
for nothing. On the other hand, whenever we apprehend an element in
the thing’s character there is less difficulty. Thus it follows
that the degree of our knowledge of a thing’s essential nature is
determined by the sense in which we are aware that it exists. Let
us then take the following as our first instance of being aware of
an element in the essential nature. Let A be eclipse, C the moon, B
the earth’s acting as a screen. Now to ask whether the moon is
eclipsed or not is to ask whether or not B has occurred. But that
is precisely the same as asking whether A has a defining condition;
and if this condition actually exists, we assert that A also
actually exists. Or again we may ask which side of a contradiction
the defining condition necessitates: does it make the angles of a
triangle equal or not equal to two right angles? When we have found
the answer, if the premisses are immediate, we know fact and reason
together; if they are not immediate, we know the fact without the
reason, as in the following example: let C be the moon, A eclipse,
B the fact that the moon fails to produce shadows though she is
full and though no visible body intervenes between us and her. Then
if B, failure to produce shadows in spite of the absence of an
intervening body, is attributable A to C, and eclipse, is
attributable to B, it is clear that the moon is eclipsed, but the
reason why is not yet clear, and we know that eclipse exists, but
we do not know what its essential nature is. But when it is clear
that A is attributable to C and we proceed to ask the reason of
this fact, we are inquiring what is the nature of B: is it the
earth’s acting as a screen, or the moon’s rotation or her
extinction? But B is the definition of the other term, viz. in
these examples, of the major term A; for eclipse is constituted by
the earth acting as a screen. Thus, (1) ‘What is thunder?’ ‘The
quenching of fire in cloud’, and (2) ‘Why does it thunder?’
‘Because fire is quenched in the cloud’, are equivalent. Let C be
cloud, A thunder, B the quenching of fire. Then B is attributable
to C, cloud, since fire is quenched in it; and A, noise, is
attributable to B; and B is assuredly the definition of the major
term A. If there be a further mediating cause of B, it will be one
of the remaining partial definitions of A.
    We have stated then how essential nature is discovered and
becomes known, and we see that, while there is no syllogism-i.e. no
demonstrative syllogism-of essential nature, yet it is through
syllogism, viz. demonstrative syllogism, that