Nor does it matter that thus to be easy and difficult to change are specific differences (of a quality), although they belong to passion and movement, and not the genus of quality.  For these differences, though apparently accidental to quality, nevertheless designate differences which are proper and essential to quality.  In the same way, in the genus of substance we often take accidental instead of substantial differences, in so far as by the former, essential principles are designated. ________________________

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.