the other false.
For 'Socrates has sight' is the opposite of 'Socrates is blind' in the
sense of the word 'opposite' which applies to possession and privation.
Now if Socrates exists, it is not necessary that one should be true and
the other false, for when he is not yet able to acquire the power of
vision, both are false, as also if Socrates is altogether non-existent.

But in the case of affirmation and negation, whether the subject exists
or not, one is always false and the other true. For manifestly, if
Socrates exists, one of the two propositions 'Socrates is ill',
'Socrates is not ill', is true, and the other false. This is likewise
the case if he does not exist; for if he does not exist, to say that he
is ill is false, to say that he is not ill is true. Thus it is in the
case of those opposites only, which are opposite in the sense in which
the term is used with reference to affirmation and negation, that the
rule holds good, that one of the pair must be true and the other false.

Part 11

That the contrary of a good is an evil is shown by induction: the
contrary of health is disease, of courage, cowardice, and so on. But
the contrary of an evil is sometimes a good, sometimes an evil. For
defect, which is an evil, has excess for its contrary, this also being
an evil, and the mean, which is a good, is equally the contrary of the
one and of the other. It is only in a few cases, however, that we see
instances of this: in most, the contrary of an evil is a good.

In the case of contraries, it is not always necessary that if one
exists the other should also exist: for if all become healthy there
will be health and no disease, and again, if everything turns white,
there will be white, but no black. Again, since the fact that Socrates
is ill is the contrary of the fact that Socrates is well, and two
contrary conditions cannot both obtain in one and the same individual
at the same time, both these contraries could not exist at once: for if
that Socrates was well was a fact, then that Socrates was ill could not
possibly be one.

It is plain that contrary attributes must needs be present in subjects
which belong to the same species or genus. Disease and health require
as their subject the body of an animal; white and black require a body,
without further qualification; justice and injustice require as their
subject the human soul.

Moreover, it is necessary that pairs of contraries should in all cases
either bel