henever a form has something proper to it besides its subject, that
form can be separate, as stated in _De Anima_ i, text. 13. Hence it
follows that a habit is a separable form; which is impossible.

Obj. 3: Further, the very notion and nature of a habit as of any
accident, is inherence in a subject: wherefore any accident is
defined with reference to its subject. Therefore if a habit does not
become more or less intense in itself, neither can it in its
inherence in its subject: and consequently it will be nowise less
intense.

_On the contrary,_ It is natural for contraries to be applicable to
the same thing. Now increase and decrease are contraries. Since
therefore a habit can increase, it seems that it can also diminish.

_I answer that,_ Habits diminish, just as they increase, in two ways,
as we have already explained (Q. 52, A. 1). And since they increase
through the same cause as that which engenders them, so too they
diminish by the same cause as that which corrupts them: since the
diminishing of a habit is the road which leads to its corruption,
even as, on the other hand, the engendering of a habit is a
foundation of its increase.

Reply Obj. 1: A habit, considered in itself, is a simple form. It is
not thus that it is subject to decrease; but according to the
different ways in which its subject participates in it. This is due
to the fact that the subject's potentiality is indeterminate, through
its being able to participate a form in various ways, or to extend to
a greater or a smaller number of things.

Reply Obj. 2: This argument would hold, if the essence itself of a
habit were nowise subject to decrease. This we do not say; but that a
certain decrease in the essence of a habit has its origin, not in the
habit, but in its subject.

Reply Obj. 3: No matter how we take an accident, its very notion
implies dependence on a subject, but in different ways. For if we
take an accident in the abstract, it implies relation to a subject,
which relation begins in the accident and terminates in the subject:
for "whiteness is that whereby a thing is white." Accordingly in
defining an accident in the abstract, we do not put the subject as
though it were the first part of the definition, viz. the genus; but
we give it the second place, which is that of the difference; thus we
say that _simitas_ is "a curvature of the nose." But if we take
accidents in the concrete, the relation begins in the subject and
terminates