But 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. ________________________

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 of an end.  Therefore it is necessary that there should be habits.

I answer that, As we have said above (AA. 2, 3), habit implies a disposition in relation to a thing’s nature, and to its operation or end, by reason of which disposition a thing is well or ill disposed thereto.  Now for a thing to need to be disposed to something else, three conditions are necessary.  The first condition is that which is disposed should be distinct from that to which it is disposed; and so, that it should be related to it as potentiality is to act.  Whence, if there is a being whose nature is not composed of potentiality and act, and whose substance is its own operation, which itself is for itself, there we can find no room for habit and disposition, as is clearly the case in God.