I was planning on writing a post on the definitions of the four cardinal Stoic virtues: practical wisdom (or prudence), justice, courage, and temperance, because discussions (and confusion) often arise about them among people interested in modern Stoicism. But Don Robertson beat me to it, and he has done such an excellent job (here) that it would be futile for me to repeat or trying to improve on his effort. (However, this post of mine may be useful; also, a good Wiki summary is here; and here are Plato’s Definitions of philosophical terms, a little known but very useful resource.)

Nonetheless, because the topic is so crucial, I am transcribing the definitions below, from Don’s post, for easy consultation by my readers. I refer you to Don’s commentary for in-depth explanations. As he points out, other than Plato’s Dictionary, the pertinent sources are the Stoic fragments from Diogenes Laertius and from Stobaeus; modern commentaries are found in Pierre Hadot’s Inner Citadel and Anthony Long’s Epictetus: A Stoic and Socratic Guide to Life. (Below, boldface indicates the bits that are particularly relevant to Stoicism.)

Phronêsis (prudence/practical wisdom)

The ability which by itself is productive of human happiness; the knowledge of what is good and bad; the knowledge that produces happiness; the disposition by which we judge what is to be done and what is not to be done.

Dikaiosynê (justice/morality)

The unanimity of the soul with itself, and the good discipline of the parts of the soul with respect to each other and concerning each other; the state that distributes to each person according to what is deserved; the state on account of which its possessor chooses what appears to him to be just; the state underlying a law-abiding way of life; social equality; the state of obedience to the laws.

Sôphrosynê (temperance/moderation)

Moderation of the soul concerning the desires and pleasures that normally occur in it; harmony and good discipline in the soul in respect of normal pleasures and pains; concord of the soul in respect of ruling and being ruled; normal personal independence; good discipline in the soul; rational agreement within the soul about what is admirable and contemptible; the state by which its possessor chooses and is cautious about what he should.

Andreia (fortitude/courage)

The state of the soul which is unmoved by fear; military confidence; knowledge of the facts of warfare; self-restraint in the soul about what is fearful and terrible; boldness in obedience to wisdom; being intrepid in the face of death; the state which stands on guard over correct thinking in dangerous situations; force which counterbalances danger; force of fortitude in respect of virtue; calm in the soul about what correct thinking takes to be frightening or encouraging things; the preservation of fearless beliefs about the terrors and experiences of warfare; the state which cleaves to the law.

BONUS I: Aretê (virtue/excellence)

The best disposition; the state of a mortal creature which is in itself praiseworthy; the state on account of which its possessor is said to be good; the just observance of the laws; the disposition on account of which he who is so disposed is said to be perfectly excellent; the state which produces faithfulness to law.

BONUS II: Eu̯dai̯monía (“happiness”/fulfillment)

The good composed of all goods; an ability which suffices for living well; perfection in respect of virtue; resources sufficient for a living creature.