t there are some habits, which even on the part of the subject in
which they are, imply primarily and principally relation to an act.
For, as we have said, habit primarily and of itself implies a
relation to the thing's nature. If therefore the nature of a thing,
in which the habit is, consists in this very relation to an act, it
follows that the habit principally implies relation to an act. Now it
is clear that the nature and the notion of power is that it should be
a principle of act. Wherefore every habit is subjected in a power,
implies principally relation to an act.

Reply Obj. 1: Habit is an act, in so far as it is a quality: and in
this respect it can be a principle of operation. It is, however, in a
state of potentiality in respect to operation. Wherefore habit is
called first act, and operation, second act; as it is explained in
_De Anima_ ii, text. 5.

Reply Obj. 2: It is not the essence of habit to be related to power,
but to be related to nature. And as nature precedes action, to which
power is related, therefore habit is put before power as a species of
quality.

Reply Obj. 3: Health is said to be a habit, or a habitual
disposition, in relation to nature, as stated above. But in so far as
nature is a principle of act, it consequently implies a relation to
act. Wherefore the Philosopher says (De Hist. Animal. x, 1), that
man, or one of his members, is called healthy, "when he can perform
the operation of a healthy man." And the same applies to other habits.
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FOURTH ARTICLE [I-II, Q. 49, Art. 4]

Whether Habits Are Necessary?

Objection 1: It would seem that habits are not necessary. For by
habits we are well or ill disposed in respect of something, as stated
above. But a thing is well or ill disposed by its form: for in
respect of its form a thing is good, even as it is a being. Therefore
there is no necessity for habits.

Obj. 2: Further, habit implies relation to an act. But power implies
sufficiently a principle of act: for even the natural powers, without
any habits, are principles of acts. Therefore there was no necessity
for habits.

Obj. 3: Further, as power is related to good and evil, so also is
habit: and as power does not always act, so neither does habit.
Given, therefore, the powers, habits become superfluous.

_On the contrary,_ Habits are perfections (Phys. vii, text. 17). But
perfection is of the greatest necessity to a thing: since it is in
the nature o