intermediate we found that it was never necessary that either the
one or the other should be present in every appropriate subject,
but only that in certain subjects one of the pair should be
present, and that in a determinate sense. It is, therefore, plain
that 'positives' and 'privatives' are not opposed each to each in
either of the senses in which contraries are opposed.

Again, in the case of contraries, it is possible that there
should be changes from either into the other, while the subject
retains its identity, unless indeed one of the contraries is a
constitutive property of that subject, as heat is of fire. For it
is possible that that that which is healthy should become
diseased, that which is white, black, that which is cold, hot,
that which is good, bad, that which is bad, good. The bad man, if
he is being brought into a better way of life and thought, may
make some advance, however slight, and if he should once improve,
even ever so little, it is plain that he might change completely,
or at any rate make very great progress; for a man becomes more
and more easily moved to virtue, however small the improvement
was at first. It is, therefore, natural to suppose that he will
make yet greater progress than he has made in the past; and as
this process goes on, it will change him completely and establish
him in the contrary state, provided he is not hindered by lack of
time. In the case of 'positives' and 'privatives', however,
change in both directions is impossible. There may be a change
from possession to privation, but not from privation to
possession. The man who has become blind does not regain his
sight; the man who has become bald does not regain his hair; the
man who has lost his teeth does not grow a new set. (iv)
Statements opposed as affirmation and negation belong manifestly