The Complete Aristotle (eng.)

upon consideration of the
opposite of the thing in question. Observe whether that opposite
has the opposite quality. If it has not, you refute the original
proposition; if it has, you establish it. E.g. ‘Temperance is
beneficial; for licentiousness is hurtful’. Or, as in the Messenian
speech, ‘If war is the cause of our present troubles, peace is what
we need to put things right again’. Or—
For if not even evil-doers should
Anger us if they meant not what they did,
Then can we owe no gratitude to such
As were constrained to do the good they did us.
    Or—
Since in this world liars may win belief,
Be sure of the opposite likewise-that this world
Hears many a true word and believes it not.
    2. Another line of proof is got by considering some modification
of the key-word, and arguing that what can or cannot be said of the
one, can or cannot be said of the other: e.g. ‘just’ does not
always mean ‘beneficial’, or ‘justly’ would always mean
‘beneficially’, whereas it is not desirable to be justly put to
death.
    3. Another line of proof is based upon correlative ideas. If it
is true that one man noble or just treatment to another, you argue
that the other must have received noble or just treatment; or that
where it is right to command obedience, it must have been right to
obey the command. Thus Diomedon, the tax-farmer, said of the taxes:
‘If it is no disgrace for you to sell them, it is no disgrace for
us to buy them’. Further, if ‘well’ or ‘justly’ is true of the
person to whom a thing is done, you argue that it is true of the
doer. But it is possible to draw a false conclusion here. It may be
just that A should be treated in a certain way, and yet not just
that he should be so treated by B. Hence you must ask yourself two
distinct questions: (1) Is it right that A should be thus treated?
(2) Is it right that B should thus treat him? and apply your
results properly, according as your answers are Yes or No.
Sometimes in such a case the two answers differ: you may quite
easily have a position like that in the Alcmaeon of Theodectes:
And was there none to loathe thy mother’s crime?
    to which question Alcmaeon in reply says,
Why, there are two things to examine here.
    And when Alphesiboea asks what he means, he rejoins:
They judged her fit to die, not me to slay her.
    Again there is the lawsuit about Demosthenes and the men who
killed Nicanor; as they were judged to have killed him justly, it
was thought that he was killed justly. And in the case of the man
who was killed at Thebes, the judges were requested to decide
whether it was unjust that he should be killed, since if it was
not, it was argued that it could not have been unjust to kill
him.
    4. Another line of proof is the ‘a fortiori’. Thus it may be
argued that if even the gods are not omniscient, certainly human
beings are not. The principle here is that, if a quality does not
in fact exist where it is more likely to exist, it clearly does not
exist where it is less likely. Again, the argument that a man who
strikes his father also strikes his neighbours follows from the
principle that, if the less likely thing is true, the more likely
thing is true also; for a man is less likely to strike his father
than to strike his neighbours. The argument, then, may run thus. Or
it may be urged that, if a thing is not true where it is more
likely, it is not true where it is less likely; or that, if it is
true where it is less likely, it is true where it is more likely:
according as we have to show that a thing is or is not true. This
argument might also be used in a case of parity, as in the
lines:
Thou hast pity for thy sire, who has lost his sons:
Hast none for Oeneus, whose brave son is dead?
    And, again, ‘if Theseus did no wrong, neither did Paris’; or
‘the sons of Tyndareus did no wrong, neither did Paris’; or ‘if
Hector did well to slay Patroclus, Paris did well to slay
Achilles’. And ‘if other followers of an art are not bad men,
neither are philosophers’. And ‘if generals are not bad men because
it often happens that they are condemned to death, neither are
sophists’. And the remark that ‘if each individual among you ought
to think of his own city’s reputation, you ought all to think of
the reputation of Greece as a whole’.
    5. Another line of argument is based on considerations of time.
Thus Iphicrates, in the case against Harmodius, said, ‘if before
doing the deed I had bargained that, if I did it,