Posterior Analytics   

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 essential nature is
exhibited. So we conclude that neither can the essential nature of
anything which has a cause distinct from itself be known without
demonstration, nor can it be demonstrated; and this is what we
contended in our preliminary discussions.

9

Now while some things have a cause distinct from themselves,
others have not. Hence it is evident that there are essential
natures which are immediate, that is are basic premisses; and of these
not only that they are but also what they are must be assumed or
revealed in some other way. This too is the actual procedure of the
arithmetician, who assumes both the nature and the existence of
unit. On the other hand, it is possible (in the manner explained) to
exhibit through demonstration the essential nature of things which
have a 'middle', i.e. a cause of their substantial being other than
that being itself; but we do not thereby demonstrate it.

10

Since definition is said to be the statement of a thing's nature,
obviously one kind of definition will be a statement of the meaning of
the name, or of an equivalent nominal formula. A definition in this
sense tells you, e.g. the meaning of the phrase 'triangular
character'. When we are aware that triangle exists, we inquire the
reason why it exists. But it is difficult thus to learn the definition
of things the existence of which we do not genuinely know-the cause of
this difficulty being, as we said before, that we only know
accidentally whether or not the thing exists. Moreover, a statement
may be a unity in either of two ways, by conjunction, like the
Iliad, or because it exhibits a single predicate as inhering not
accidentally in a single subject.
That then is one way of defining definition. Another kind of
definition is a formula exhibiting the cause of a thing's existence.
Thus the former signifies without proving, but the latter will clearly
be a quasi-demonstration of essential nature, differing from
demonstration in the arrangement of its terms. For there is a
difference between stating why it thunders, and stating what is the
essential nature of thunder; since the first statement will be
'Because fire is quenched in the clouds', while the statement of
what the nature of thunder is will be 'The noise of fire being
quenched in the clouds'. Thus the same statement takes a different
form: in one form it is continuous demonstration, in the other
definition. Again, thunder can be defined as noise in the clouds,
which is the conclusion of the demonstration embodying essential
nature. On the other hand the definition of immediates is an