ntal instead of
substantial differences, in so far as by the former, essential
principles are designated.
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THIRD ARTICLE [I-II, Q. 49, Art. 3]

Whether Habit Implies Order to an Act?

Objection 1: It would seem that habit does not imply order to an act.
For everything acts according as it is in act. But the Philosopher
says (De Anima iii, text 8), that "when one is become knowing by
habit, one is still in a state of potentiality, but otherwise than
before learning." Therefore habit does not imply the relation of a
principle to an act.

Obj. 2: Further, that which is put in the definition of a thing,
belongs to it essentially. But to be a principle of action, is put in
the definition of power, as we read in _Metaph._ v, text. 17.
Therefore to be the principle of an act belongs to power essentially.
Now that which is essential is first in every genus. If therefore,
habit also is a principle of act, it follows that it is posterior to
power. And so habit and disposition will not be the first species of
quality.

Obj. 3: Further, health is sometimes a habit, and so are leanness and
beauty. But these do not indicate relation to an act. Therefore it is
not essential to habit to be a principle of act.

_On the contrary,_ Augustine says (De Bono Conjug. xxi) that "habit
is that whereby something is done when necessary." And the
Commentator says (De Anima iii) that "habit is that whereby we act
when we will."

_I answer that,_ To have relation to an act may belong to habit, both
in regard to the nature of habit, and in regard to the subject in
which the habit is. In regard to the nature of habit, it belongs to
every habit to have relation to an act. For it is essential to habit
to imply some relation to a thing's nature, in so far as it is
suitable or unsuitable thereto. But a thing's nature, which is the
end of generation, is further ordained to another end, which is
either an operation, or the product of an operation, to which one
attains by means of operation. Wherefore habit implies relation not
only to the very nature of a thing, but also, consequently, to
operation, inasmuch as this is the end of nature, or conducive to the
end. Whence also it is stated (Metaph. v, text. 25) in the definition
of habit, that it is a disposition whereby that which is disposed, is
well or ill disposed either in regard to itself, that is to its
nature, or in regard to something else, that is to the end.

Bu