Aristotle says little in detail of the dianoetic virtues. And we may turn at once to the main subject of his moral system, the ethical virtues. These consist in the governance of the passions by reason. Socrates was wrong in supposing that virtue is purely intellectual, that nothing save knowledge is needed for it, and that if a man thinks right he must needs do right. He forgot the existence of the passions, which are not easily controlled. A man may reason perfectly, his reason may point him to the right path, but his passions may get the upper hand and lead him out of it. How then is reason to gain control over the appetites? Only by practice. It is only by continual effort, by the constant exercise of self-control, that the unruly passions can be tamed. Once brought under the yoke, their control becomes habit. Aristotle lays the utmost emphasis on the importance of habit in morality. It is only by cultivating good habits that a man becomes good. Now if virtue consists in the control of the appetites by reason, it thus contains two constituents, reason and appetite. Both must be present. There must be passions, if they are to be controlled. Hence the ascetic ideal of rooting out the passions altogether is fundamentally wrong. It overlooks the fact that the higher form does not exclude the lower--that were contrary to the conception of evolution--it includes and transcends it. It forgets that the passions are an organic part of man, and that to destroy them is to do injury to his {318} nature by destroying one of its essential members. The passions and appetites are, in fact, the matter of virtue, reason its form, and the mistake of asceticism is that it destroys the matter of virtue, and supposes that the form can subsist by itself. Virtue means that the appetites must be brought under control, not that they must be eradicated. Hence there are two extremes to be avoided. It is extreme, on the one hand, to attempt to uproot the passions; and it is extreme, on the other, to allow them to run riot. Virtue means moderation. It consists in hitting the happy mean as regards the passions, in not allowing them to get the upper hand of reason, and yet in not being quite passionless and apathetic. From this follows the famous Aristotelian doctrine of virtue as the mean between two extremes. Every virtue lies between two vices, which are the excess and defect of appetite respectively.