Posterior Analytics   

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 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