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 belong to the same genus or belong to contrary
genera or be themselves genera. White and black belong to the
same genus, colour; justice and injustice, to contrary genera,
virtue and vice; while good and evil do not belong to genera, but
are themselves actual genera, with terms under them.