The Complete Aristotle (eng.)

that contraries are
not subjects of the same science, his opponent must reply that
there is a single science of all opposites. Thus we must have the
first figure: for the term which embraces the original subject
becomes the middle term.
    If the objection is particular, the objector must frame his
contradiction with reference to a term relatively to which the
subject of his opponent’s premiss is universal, e.g. he will point
out that the knowable and the unknowable are not subjects of the
same science: ‘contraries’ is universal relatively to these. And we
have the third figure: for the particular term assumed is middle,
e.g. the knowable and the unknowable. Premisses from which it is
possible to draw the contrary conclusion are what we start from
when we try to make objections. Consequently we bring objections in
these figures only: for in them only are opposite syllogisms
possible, since the second figure cannot produce an affirmative
conclusion.
    Besides, an objection in the middle figure would require a
fuller argument, e.g. if it should not be granted that A belongs to
B, because C does not follow B. This can be made clear only by
other premisses. But an objection ought not to turn off into other
things, but have its new premiss quite clear immediately. For this
reason also this is the only figure from which proof by signs
cannot be obtained.
    We must consider later the other kinds of objection, namely the
objection from contraries, from similars, and from common opinion,
and inquire whether a particular objection cannot be elicited from
the first figure or a negative objection from the second.
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    A probability and a sign are not identical, but a probability is
a generally approved proposition: what men know to happen or not to
happen, to be or not to be, for the most part thus and thus, is a
probability, e.g. ‘the envious hate’, ‘the beloved show affection’.
A sign means a demonstrative proposition necessary or generally
approved: for anything such that when it is another thing is, or
when it has come into being the other has come into being before or
after, is a sign of the other’s being or having come into being.
Now an enthymeme is a syllogism starting from probabilities or
signs, and a sign may be taken in three ways, corresponding to the
position of the middle term in the figures. For it may be taken as
in the first figure or the second or the third. For example the
proof that a woman is with child because she has milk is in the
first figure: for to have milk is the middle term. Let A represent
to be with child, B to have milk, C woman. The proof that wise men
are good, since Pittacus is good, comes through the last figure.
Let A stand for good, B for wise men, C for Pittacus. It is true
then to affirm both A and B of C: only men do not say the latter,
because they know it, though they state the former. The proof that
a woman is with child because she is pale is meant to come through
the middle figure: for since paleness follows women with child and
is a concomitant of this woman, people suppose it has been proved
that she is with child. Let A stand for paleness, B for being with
child, C for woman. Now if the one proposition is stated, we have
only a sign, but if the other is stated as well, a syllogism, e.g.
‘Pittacus is generous, since ambitious men are generous and
Pittacus is ambitious.’ Or again ‘Wise men are good, since Pittacus
is not only good but wise.’ In this way then syllogisms are formed,
only that which proceeds through the first figure is irrefutable if
it is true (for it is universal), that which proceeds through the
last figure is refutable even if the conclusion is true, since the
syllogism is not universal nor correlative to the matter in
question: for though Pittacus is good, it is not therefore
necessary that all other wise men should be good. But the syllogism
which proceeds through the middle figure is always refutable in any
case: for a syllogism can never be formed when the terms are
related in this way: for though a woman with child is pale, and
this woman also is pale, it is not necessary that she should be
with child. Truth then may be found in signs whatever their kind,
but they have the differences we have stated.
    We must either divide signs in the way stated, and among them
designate the middle term as the index (for people call that the
index which makes us know, and the middle term above all has this
character), or else we must call the arguments