Posterior Analytics   

proceeding to ask the reason of the connexion or the nature of the
thing, then we are asking what the 'middle' is.
(By distinguishing the fact of the connexion and the existence of
the thing as respectively the partial and the unqualified being of the
thing, I mean that if we ask 'does the moon suffer eclipse?', or 'does
the moon wax?', the question concerns a part of the thing's being; for
what we are asking in such questions is whether a thing is this or
that, i.e. has or has not this or that attribute: whereas, if we ask
whether the moon or night exists, the question concerns the
unqualified being of a thing.)
We conclude that in all our inquiries we are asking either whether
there is a 'middle' or what the 'middle' is: for the 'middle' here
is precisely the cause, and it is the cause that we seek in all our
inquiries. Thus, 'Does the moon suffer eclipse?' means 'Is there or is
there not a cause producing eclipse of the moon?', and when we have
learnt that there is, our next question is, 'What, then, is this
cause? for the cause through which a thing is-not is this or that,
i.e. has this or that attribute, but without qualification is-and
the cause through which it is-not is without qualification, but is
this or that as having some essential attribute or some accident-are
both alike the middle'. By that which is without qualification I
mean the subject, e.g. moon or earth or sun or triangle; by that which
a subject is (in the partial sense) I mean a property, e.g. eclipse,
equality or inequality, interposition or non-interposition. For in all
these examples it is clear that the nature of the thing and the reason
of the fact are identical: the question 'What is eclipse?' and its
answer 'The privation of the moon's light by the interposition of
the earth' are identical with the question 'What is the reason of
eclipse?' or 'Why does the moon suffer eclipse?' and the reply
'Because of the failure of light through the earth's shutting it out'.
Again, for 'What is a concord? A commensurate numerical ratio of a
high and a low note', we may substitute 'What ratio makes a high and a
low note concordant? Their relation according to a commensurate
numerical ratio.' 'Are the high and the low note concordant?' is
equivalent to 'Is their ratio commensurate?'; and when we find that it
is commensurate, we ask 'What, then, is their ratio?'.
Cases in which the 'middle' is sensible show that the object of
our inquiry is always the 'middle': we inquire, because we have not
perceived it, whether there is or is not a 'middle' causing, e.g. an
eclipse. On the other hand, if we were on the moon we should not be
inquiring either as to the fact or the reason, but both fact and
reason would be obvious simultaneously. For the act of perception
would have enabled us to know the universal too; since, the present
fact of an eclipse being evident, perception would then at the same
time give us the present fact of the earth's screening the sun's
light, and from this would arise the universal.
Thus, as we maintain, to know a thing's nature is to know the reason
why it is; and this is equally true of things in so far as they are
said without qualification to he as opposed to being possessed of some
attribute, and in so far as they are said to be possessed of some
attribute such as equal to right angles, or greater or less.

3

It is clear, then, that all questions are a search for a 'middle'.
Let us now state how essential nature is revealed and in what way it
can be reduced to demonstration; what definition is, and what things
are definable. And let us first discuss certain difficulties which
these questions raise, beginning what we have to say with a point most
intimately connected with our immediately preceding remarks, namely
the doubt that might be felt as to whether or not it is possible to
know the same thing in the same relation, both by definition and by
demonstration. It might, I mean, be urged that definition is held to
concern essential nature and is in every case universal and