Most meditation trains what the mind rests on. Mettā trains what the mind rests as. The word is usually translated loving-kindness: an unconditional friendliness toward beings, starting with the one you find hardest to befriend. The root text, the Karaṇīya Mettā Sutta, compresses the whole practice into one image. As a mother protects her only child with her life, so should one cultivate a boundless heart toward all beings, above, below, and all around, without obstruction, without enmity.
The practice
The classical method, systematized in the Visuddhimagga, works in widening circles. Sit, settle the breath, and begin by directing a few simple phrases toward yourself: may I be safe; may I be well; may I be at ease. The phrases are not magic words. They are the handle of an intention. Say them slowly enough to mean them.
Then widen, circle by circle, holding each person in mind as if they sat before you. A benefactor (someone who has been good to you, the easiest heart-opener). A dear friend. A neutral person (the barista, the stranger on the bus; this circle is where the practice becomes real). Then, gently, and only as far as is honest, a difficult person. End boundlessly: all beings, everywhere, without exception. Ten to twenty minutes. The order can flex. The widening is the point.
What it trains
Of the five trainings, this is the school of the heart, and the tradition is blunt that warmth is a trainable skill, not a temperament some are born with. The modern record agrees further than one might expect. In a randomized workplace trial, Barbara Fredrickson and colleagues found that weeks of loving-kindness practice increased daily positive emotion, which in turn built durable personal resources: attention, purpose, social closeness, even physical symptoms improving. The mechanism they proposed, small repeated emotions compounding into life-structure, is precisely the tradition’s claim in laboratory dress.
Cautions and honesty
The difficult-person circle is not a demand for instant forgiveness. Forcing it breeds pretense, and the tradition would rather you stop at the honest edge. For some, particularly those carrying harsh self-criticism or trauma, beginning with oneself is the hardest circle. It is permitted, and traditional, to begin with the benefactor instead and return to yourself when the heart has warmed.