The Complete Aristotle (eng.)

Form present
in each individual.
    Again, if there be posited an accident which has a contrary,
look and see if that which admits of the accident will admit of its
contrary as well: for the same thing admits of contraries. Thus
(e.g.) if he has asserted that hatred follows anger, hatred would
in that case be in the ‘spirited faculty’: for that is where anger
is. You should therefore look and see if its contrary, to wit,
friendship, be also in the ‘spirited faculty’: for if not-if
friendship is in the faculty of desire-then hatred could not follow
anger. Likewise also if he has asserted that the faculty of desire
is ignorant. For if it were capable of ignorance, it would be
capable of knowledge as well: and this is not generally held-I mean
that the faculty of desire is capable of knowledge. For purposes,
then, of overthrowing a view, as has been said, this rule should be
observed: but for purposes of establishing one, though the rule
will not help you to assert that the accident actually belongs, it
will help you to assert that it may possibly belong. For having
shown that the thing in question will not admit of the contrary of
the accident asserted, we shall have shown that the accident
neither belongs nor can possibly belong; while on the other hand,
if we show that the contrary belongs, or that the thing is capable
of the contrary, we shall not indeed as yet have shown that the
accident asserted does belong as well; our proof will merely have
gone to this point, that it is possible for it to belong.
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8
    Seeing that the modes of opposition are four in number, you
should look for arguments among the contradictories of your terms,
converting the order of their sequence, both when demolishing and
when establishing a view, and you should secure them by means of
induction-such arguments (e.g.) as that man be an animal, what is
not an animal is not a man’: and likewise also in other instances
of contradictories. For in those cases the sequence is converse:
for ‘animal’ follows upon ‘man but ‘not-animal’ does not follow
upon ‘not-man’, but conversely ‘not-man’ upon ‘not-animal’. In all
cases, therefore, a postulate of this sort should be made, (e.g.)
that ‘If the honourable is pleasant, what is not pleasant is not
honourable, while if the latter be untrue, so is the former’.
Likewise, also, ‘If what is not pleasant be not honourable, then
what is honourable is pleasant’. Clearly, then, the conversion of
the sequence formed by contradiction of the terms of the thesis is
a method convertible for both purposes.
    Then look also at the case of the contraries of S and P in the
thesis, and see if the contrary of the one follows upon the
contrary of the other, either directly or conversely, both when you
are demolishing and when you are establishing a view: secure
arguments of this kind as well by means of induction, so far as may
be required. Now the sequence is direct in a case such as that of
courage and cowardice: for upon the one of them virtue follows, and
vice upon the other; and upon the one it follows that it is
desirable, while upon the other it follows that it is
objectionable. The sequence, therefore, in the latter case also is
direct; for the desirable is the contrary of the objectionable.
Likewise also in other cases. The sequence is, on the other hand,
converse in such a case as this: Health follows upon vigour, but
disease does not follow upon debility; rather debility follows upon
disease. In this case, then, clearly the sequence is converse.
Converse sequence is, however, rare in the case of contraries;
usually the sequence is direct. If, therefore, the contrary of the
one term does not follow upon the contrary of the other either
directly or conversely, clearly neither does the one term follow
upon the other in the statement made: whereas if the one followed
the other in the case of the contraries, it must of necessity do so
as well in the original statement.
    You should look also into cases of the privation or presence of
a state in like manner to the case of contraries. Only, in the case
of such privations the converse sequence does not occur: the
sequence is always bound to be direct: e.g. as sensation follows
sight, while absence of sensation follows blindness. For the
opposition of sensation to absence of sensation is an opposition of
the presence to the privation of a state: for the one of them is a
state, and the other