How this question affects virtues we shall state further on (Q. 66, A. 1).

Reply Obj. 1:  Even in bodily bulk increase is twofold.  First, by addition of one subject to another; such is the increase of living things.  Secondly, by mere intensity, without any addition at all; such is the case with things subject to rarefaction, as is stated in Phys. iv, text. 63.

Reply Obj. 2:  The cause that increases a habit, always effects something in the subject, but not a new form.  But it causes the subject to partake more perfectly of a pre-existing form, or it makes the form to extend further.

Reply Obj. 3:  What is not already white, is potentially white, as not yet possessing the form of whiteness:  hence the agent causes a new form in the subject.  But that which is less hot or white, is not in potentiality to those forms, since it has them already actually:  but it is in potentiality to a perfect mode of participation; and this it receives through the agent’s action. ________________________

THIRD ARTICLE [I-II, Q. 52, Art. 3]

Whether Every Act Increases Its Habit?

Objection 1:  It would seem that every act increases its habit.  For when the cause is increased the effect is increased.  Now acts are causes of habits, as stated above (Q. 51, A. 2).  Therefore a habit increases when its acts are multiplied.

Obj. 2:  Further, of like things a like judgment should be formed.  But all the acts proceeding from one and the same habit are alike (Ethic. ii, 1, 2).  Therefore if some acts increase a habit, every act should increase it.

Obj. 3:  Further, like is increased by like.  But any act is like the habit whence it proceeds.  Therefore every act increases the habit.

On the contrary, Opposite effects do not result from the same cause.  But according to Ethic. ii, 2, some acts lessen the habit whence they proceed, for instance if they be done carelessly.  Therefore it is not every act that increases a habit.

I answer that, “Like acts cause like habits” (Ethic. ii, 1, 2).  Now things are like or unlike not only in respect of their qualities being the same or various, but also in respect of the same or a different mode of participation.  For it is not only black that is unlike white, but also less white is unlike more white, since there is movement from less white to more white, even as from one opposite to another, as stated in Phys. v, text. 52.

But since use of habits depends on the will, as was shown above (Q. 50, A. 5); just as one who has a habit may fail to use it or may act contrary to it; so may he happen to use the habit by performing an act that is not in proportion to the intensity of the habit.  Accordingly, if the intensity of the act correspond in proportion to the intensity of the habit, or even surpass it, every such act either increases the habit or disposes to an increase thereof, if we may speak