 The moral theory of Aristotle, like that of Plato, focuses on virtue, recommending the virtuous way of life by its relation to happiness. His most important ethical work, Nicomachean Ethics, devotes the first book to a preliminary account of happiness, which is then completed in the last chapters of the final book, Book X. This account ties happiness to excellent activity of the soul. In subsequent books, excellent activity of the soul is tied to the moral virtues and to the virtue of “practical wisdom” – excellence in thinking and deciding about how to behave. This approach to moral theory depends on a moral psychology that shares a number of affinities with Plato’s. However, while for Plato the theory of forms has a role in justifying virtue, Aristotle notoriously rejects that theory. Aristotle grounds his account of virtue in his theory about the soul – a topic to which he devotes a separate treatise, de Anima.