The Complete Aristotle (eng.)

judgement that that which is good is good.
Besides these there is the judgement that that which is good is not
good, parallel with the judgement that that that is not good is
good. Let us consider, therefore, what would form the contrary of
the true judgement that that which is not good is not good. The
judgement that it is bad would, of course, fail to meet the case,
since two true judgements are never contrary and this judgement
might be true at the same time as that with which it is connected.
For since some things which are not good are bad, both judgements
may be true. Nor is the judgement that it is not bad the contrary,
for this too might be true, since both qualities might be
predicated of the same subject. It remains, therefore, that of the
judgement concerning that which is not good, that it is not good,
the contrary judgement is that it is good; for this is false. In
the same way, moreover, the judgement concerning that which is
good, that it is not good, is the contrary of the judgement that it
is good.
    It is evident that it will make no difference if we universalize
the positive judgement, for the universal negative judgement will
form the contrary. For instance, the contrary of the judgement that
everything that is good is good is that nothing that is good is
good. For the judgement that that which is good is good, if the
subject be understood in a universal sense, is equivalent to the
judgement that whatever is good is good, and this is identical with
the judgement that everything that is good is good. We may deal
similarly with judgements concerning that which is not good.
    If therefore this is the rule with judgements, and if spoken
affirmations and denials are judgements expressed in words, it is
plain that the universal denial is the contrary of the affirmation
about the same subject. Thus the propositions ‘everything good is
good’, ‘every man is good’, have for their contraries the
propositions ‘nothing good is good’, ‘no man is good’. The
contradictory propositions, on the other hand, are ‘not everything
good is good’, ‘not every man is good’.
    It is evident, also, that neither true judgements nor true
propositions can be contrary the one to the other. For whereas,
when two propositions are true, a man may state both at the same
time without inconsistency, contrary propositions are those which
state contrary conditions, and contrary conditions cannot subsist
at one and the same time in the same subject.

Prior Analytics, Book I
    Translated by A. J. Jenkinson
1
    We must first state the subject of our inquiry and the faculty
to which it belongs: its subject is demonstration and the faculty
that carries it out demonstrative science. We must next define a
premiss, a term, and a syllogism, and the nature of a perfect and
of an imperfect syllogism; and after that, the inclusion or
noninclusion of one term in another as in a whole, and what we mean
by predicating one term of all, or none, of another.
    A premiss then is a sentence affirming or denying one thing of
another. This is either universal or particular or indefinite. By
universal I mean the statement that something belongs to all or
none of something else; by particular that it belongs to some or
not to some or not to all; by indefinite that it does or does not
belong, without any mark to show whether it is universal or
particular, e.g. ‘contraries are subjects of the same science’, or
‘pleasure is not good’. The demonstrative premiss differs from the
dialectical, because the demonstrative premiss is the assertion of
one of two contradictory statements (the demonstrator does not ask
for his premiss, but lays it down), whereas the dialectical premiss
depends on the adversary’s choice between two contradictories. But
this will make no difference to the production of a syllogism in
either case; for both the demonstrator and the dialectician argue
syllogistically after stating that something does or does not
belong to something else. Therefore a syllogistic premiss without
qualification will be an affirmation or denial of something
concerning something else in the way we have described; it will be
demonstrative, if it is true and obtained through the first
principles of its science; while a dialectical premiss is the
giving of a choice between two contradictories, when a man is
proceeding by question, but when he is syllogizing it is the
assertion of that which is apparent and generally admitted, as has
been said