The Complete Aristotle (eng.)

contradictories of these
contraries will both be true, as in the instance before us: the
propositions ‘not every animal is just’ and ‘some animals are just’
are both true.
    Further, the proposition ‘no man is just’ follows from the
proposition ‘every man is not just’ and the proposition ‘not every
man is not just’, which is the opposite of ‘every man is not-just’,
follows from the proposition ‘some men are just’; for if this be
true, there must be some just men.
    It is evident, also, that when the subject is individual, if a
question is asked and the negative answer is the true one, a
certain positive proposition is also true. Thus, if the question
were asked Socrates wise?’ and the negative answer were the true
one, the positive inference ‘Then Socrates is unwise’ is correct.
But no such inference is correct in the case of universals, but
rather a negative proposition. For instance, if to the question ‘Is
every man wise?’ the answer is ‘no’, the inference ‘Then every man
is unwise’ is false. But under these circumstances the inference
‘Not every man is wise’ is correct. This last is the contradictory,
the former the contrary. Negative expressions, which consist of an
indefinite noun or predicate, such as ‘not-man’ or ‘not-just’, may
seem to be denials containing neither noun nor verb in the proper
sense of the words. But they are not. For a denial must always be
either true or false, and he that uses the expression ‘not man’, if
nothing more be added, is not nearer but rather further from making
a true or a false statement than he who uses the expression
‘man’.
    The propositions ‘everything that is not man is just’, and the
contradictory of this, are not equivalent to any of the other
propositions; on the other hand, the proposition ‘everything that
is not man is not just’ is equivalent to the proposition ‘nothing
that is not man is just’.
    The conversion of the position of subject and predicate in a
sentence involves no difference in its meaning. Thus we say ‘man is
white’ and ‘white is man’. If these were not equivalent, there
would be more than one contradictory to the same proposition,
whereas it has been demonstrated’ that each proposition has one
proper contradictory and one only. For of the proposition ‘man is
white’ the appropriate contradictory is ‘man is not white’, and of
the proposition ‘white is man’, if its meaning be different, the
contradictory will either be ‘white is not not-man’ or ‘white is
not man’. Now the former of these is the contradictory of the
proposition ‘white is not-man’, and the latter of these is the
contradictory of the proposition ‘man is white’; thus there will be
two contradictories to one proposition.
    It is evident, therefore, that the inversion of the relative
position of subject and predicate does not affect the sense of
affirmations and denials.
11
    There is no unity about an affirmation or denial which, either
positively or negatively, predicates one thing of many subjects, or
many things of the same subject, unless that which is indicated by
the many is really some one thing. do not apply this word ‘one’ to
those things which, though they have a single recognized name, yet
do not combine to form a unity. Thus, man may be an animal, and
biped, and domesticated, but these three predicates combine to form
a unity. On the other hand, the predicates ‘white’, ‘man’, and
‘walking’ do not thus combine. Neither, therefore, if these three
form the subject of an affirmation, nor if they form its predicate,
is there any unity about that affirmation. In both cases the unity
is linguistic, but not real.
    If therefore the dialectical question is a request for an
answer, i.e. either for the admission of a premiss or for the
admission of one of two contradictories-and the premiss is itself
always one of two contradictories-the answer to such a question as
contains the above predicates cannot be a single proposition. For
as I have explained in the Topics, question is not a single one,
even if the answer asked for is true.
    At the same time it is plain that a question of the form ‘what
is it?’ is not a dialectical question, for a dialectical questioner
must by the form of his question give his opponent the chance of
announcing one of two alternatives, whichever he wishes. He must
therefore put the question into a more definite form, and inquire,
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