Friendliness (metta), is the first of the four cardinal virtues of Theravada Buddhism. “Mindfulness of friendliness” is among the daily exercises of the monk and can also be practiced by the layman; the practitioner detaches himself in imagination from his own body and, as though looking down on himself, pervades himself with friendliness directed toward himself, for it is impossible to feel true friendliness or love for others unless, in the best sense of the term, one feels it for oneself; then he proceeds in imagination to send waves of friendliness in every direction, to reach every being in every corner of the world After pervading the world with love, he may repeat the process with the three other cardinal virtues—compassion, joy, and equanimity. These forms of the practice of “right mindfulness” are known as brahma-viharas, freely translated “sublime moods.” They are still practiced by Buddhists throughout the world, and it is believed, especially among the Mahayanist sects, that the waves of friendliness constantly poured out by many thousands of meditating monks have a very positive effect on the welfare of the world.
The foregoing comes from Ainslie T. Embree, ed., Sources of Indian Tradition, Volume One: From the Beginning to 1800, second edition (New York: Columbia University Press, 1988), p. 118.