 Knowledge, then, about what is good, bad, and indifferent is the heart of virtue. Courage, e.g., is simply knowledge of what is to be endured: the impulse to endure or not, and the only impulse that is needed by courage, then follows automatically, as a product or aspect of that knowledge. This tight unity in the soul is the basis for the Stoic teaching about the unity of the virtues. Zeno (the founder of the school) defines wisdom (phronêsis), or rather practical knowledge, in matters requiring distribution as justice, in matters requiring choice as moderation, and in matters requiring endurance as courage (Plutarch On Moral Virtue 440E–441D). Practical knowledge, then, is a single, comprehensive knowledge of what is good and bad in each of these kinds of circumstance.