The second of the five carita-viśodhana practices, corresponding to the dveṣa-carita (hatred-temperament) practitioner. The Śrāvakabhūmi treats maitrī in its definitional aspect — three marks (mitra-pakṣa, friend-side; amitra-pakṣa, enemy-side; udāsīna-pakṣa, neutral-side), three intensities, the brahma-vihāra division of labor across the four immeasurables, and the temperament rationale for why hatred yields specifically to the cultivation of friendliness. Procedural development of the practice itself is given separately in the vipaśyanā illustration in sbh3.2.
What is lovingkindness?
Toward the friend-side, the enemy-side, or the neutral-side, having established the benefit-aspiration, [it is] the resolve to bring mild, middling, or strong happiness — a resolve grounded in the concentrated plane. The friend-side, enemy-side, and neutral-side — these are the object. The benefit-aspiration and the resolve to bring happiness, grounded in the concentrated plane — this is the operator. The object and the operator, combined into one — this is called lovingkindness.
When [the canonical formula] says 'with mind imbued with lovingkindness,' by this the benefit-aspiration is named, [extending] through the three sides — friend, enemy, neutral. When it further says: 'without enmity, without opposition, without harming' — by this, the threefold characteristic of that benefit-aspiration is named. Of these: by non-enmity, the benefit-aspiration [is characterized]. This non-enmity is named by two further terms: non-opposition and non-harming. Non-opposition is so named because it has the sense of being-without-an-opposing-stance. Non-harming is so named because it has the sense of being-without-malice-and-injury.
When it further says: 'vast, exalted, immeasurable' — by this, the bringing of mild, middling, or strong happiness is named: happiness of the desire-realm, or of the first-and-second-absorption planes, or of the third-absorption plane. When it further says: [Damaged/Conjectural]: 'having attained, he abides resolved upon' — by this, the resolve to bring happiness, grounded in the concentrated plane, is named.
This bringing of happiness, encompassed by the benefit-aspiration, characterized by resolve, accompanied by attention — should be understood as directed toward those who are neither-suffering-nor-happy, desiring happiness, on the friend-side, enemy-side, or neutral-side. Whereas for the one who is suffering, or the non-suffering [that is, happy] one, on the friend-side, the enemy-side, or the neutral-side: the suffering one is the object of compassion; the happy one is the object of sympathetic joy. This is called lovingkindness.
The ill-will-conduct person, cultivating lovingkindness, attenuates the ill-will toward beings. He purifies the mind from ill-will.