For the disciplined, the Gita says, the mind is the best of friends; for the undisciplined, it is the worst of enemies. These verses describe moderation, steadiness, and the quiet work of self-mastery.
One should raise oneself by one's Self alone; let not one lower oneself; for the Self alone is the friend of oneself, and the Self alone is the enemy of oneself.
Verily Yoga is not possible for him who eats too much, nor for him who does not eat at all, nor for him who sleeps too much, nor for him who is (always) awake, O Arjuna.
Yoga becomes the destroyer of pain for him who is moderate in eating and recreation (such
as walking, etc.), who is moderate in exertion in actions, who is moderate in sleep and wakefulness.
Worship of the gods, the twice-born, the teachers and the wise, purity, straightforwardness, celibacy and non-injury are called the austerities of the body.
Speech which causes no excitement, truthful, pleasant and beneficial, the practice of the study of the Vedas, are called austerity of speech.
Serenity of mind, good-heartedness, self-control, purity of nature this is called mental austerity.