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 FP, 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:

A1 Whether habit is a quality?

A2 Whether it is a distinct species of quality?

A3 Whether habit implies an order to an act?

A4 Of the necessity of habit.