After treating of human acts and passions, we now pass on to the consideration of the principles of human acts, and firstly of intrinsic principles, secondly of extrinsic principles. The intrinsic principle is power and habit; but as we have treated of powers in the First Part (Q. 77, seqq.), it remains for us to consider them in general: in the second place we shall consider virtues and vices and other like habits, which are the principles of human acts.
Concerning habits in general there are four points to consider: First, the substance of habits; second, their subject; third, the cause of their generation, increase, and corruption; fourth, how they are distinguished from one another.
Under the first head, there are four points of inquiry:
(1) Whether habit is a quality?
(2) Whether it is a distinct species of quality?
(3) Whether habit implies an order to an act?
(4) Of the necessity of habit. ________________________
FIRST ARTICLE [I-II, Q. 49, Art. 1]
Whether Habit Is a Quality?
Objection 1: It would seem that habit is not a quality. For Augustine says (QQ. lxxxiii, qu. 73): "this word 'habit' is derived from the verb 'to have.'" But "to have" belongs not only to quality, but also to the other categories: for we speak of ourselves as "having" quantity and money and other like things. Therefore habit is not a quality.