142. Definition.—A virtue is a good habit of the free powers of the soul, that is a principle of good conduct, and never of conduct that is evil. Hence, the following are not virtues: (a) an occasional inclination to good, for this is not a fixed habit; (b) good habits of the body or of the vegetative powers, etc. (such as beauty and health), for these are not free; (c) knowledge of the right or affection for it without any reference to practice, for virtue is a principle of right living; (d) habits that can be applied indifferently to good or bad conduct, such as human opinion.
143. Division.—The virtues are divided: (a) according to their different causes, into infused and acquired virtues (cfr. 134 sqq.);(b) according to their different objects, into intellectual, moral and theological virtues.
144. The intellectual virtues are those habits that perfect the intellect with reference to its good—i.e., truth, speculative or practical.
145. The speculative virtues are three: understanding, knowledge and wisdom.
(a) Understanding or intelligence is the habit of perceiving truths that are not in need of proof, as being self-evident. Axiomatic truths or first principles are the object of this virtue.
(b) Knowledge or science is the habit of perceiving truths that are learned from other truths by argumentation, and that are ultimate in some category of being. The object of this virtue embraces the various sciences (like astronomy) which are conclusions from principles.
(c) Wisdom is the habit of learning through reasoning the truth that is absolutely ultimate; it is the knowledge of things in their supreme cause, God. Examples are theology and philosophy in their highest sense.
146. The practical intellectual virtues are two: prudence and art.
(a) Prudence is an intellectual virtue which indicates in individual cases what is to be done or what is to be omitted, in order that one may act according to the requirements of good morals.
(b) Art is an intellectual virtue which indicates in individual cases how one must act in order to produce things that are useful or beautiful (e.g., music, painting, building, etc.).