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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.






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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