The knower of Brahman is neither lifted by what is pleasant nor sunk by what is unpleasant.

Advaita Vedānta

These commentators ground the steadiness in non-dual vision: the knower sees the one same faultless Self in all beings, so there is literally no distinct dear or undear thing apart from himself to be gained, and therefore no occasion for joy or grief at all. They map the four marks onto a graded ripening of practice: steady understanding comes from renunciation-preceded inquiry and the maturing of hearing and reflection; freedom from delusion comes from the ripening of meditation, an uninterrupted flow of homogeneous Self-cognition; and settledness in Brahman comes from the ripening of absorption. Such a knower, established as 'I am Brahman,' is a renouncer who stands in Brahman doing no action.

Śaṅkarācārya · Śrī Ānandagiri · Madhusūdana Sarasvatī · Śrī Nīlakaṇṭha · Dhanapati Sūri

Viśiṣṭādvaita

These commentators do not erase the objects but trace the dear and undear back to the body and its states, which are themselves limiting conditions (upadhi) shaped by past karma. The qualification 'whatever body in whatever state' brings out that pleasure and pain belong to the body, not to the Self. Delusion here is precisely the bewilderment that confuses the steady Self with the unsteady body; the knower, taught by knowers of truth, gives up the conceit of the body and rests in the experience of the Self of steady form. The four predicates are one inner state seen from four sides, and being a knower of Brahman is joined with the active practice of that Brahman.

Rāmānujācārya · Vedānta Deśika

Bhedabheda

This commentator reads the prohibition with a lighter touch, taking the prefix in na prahrishyet to mean 'not exceedingly': one should not rejoice exceedingly on gaining the desired, since excessive elation is not to be indulged, and one should not be agitated on the undesired, agitation being a distress of mind. He glosses the undeluded one as having discriminating mental activity, and points forward to the happiness within the Self that one untouched by outer contacts enjoys, an imperishable happiness from union with Brahman.

Śrī Bhāskara

Dvaita

These commentators read the verse less as a standalone portrait and more as a structural signpost. The point made is that from here to the end of the chapter Krishna unfolds renunciation, yoga, and knowledge together, joined as they are actually grasped and not split into separate topics. They reject the views that the chapter's remainder is only about the knower's absence of action, or only about the nature of knowledge and its means. By taking up renunciation at the outset, the verse signals that 'let him not rejoice' is itself an expansion of renunciation.

Madhvācārya · Śrī Jayatīrtha

Śuddhādvaita

These commentators read the steadiness through the joy of the indwelling Lord. The yogi's gladness is the joy of inner stilling, the sattvika happiness found within the Self; the indwelling Purushottama is felt within, and the soul's response is a steady gladness that no outer gain can swell and no outer loss can shrink. One reading adds a striking turn on the unpleasant: separation has been given by Bhagavan for the sake of bestowing supreme happiness, so one should not be agitated at it, for agitation would block the further attainment; and one should not exult at gaining the dear either, since exultation breeds pride and so a later fault.

Vallabhācārya · Śrī Puruṣottama

Kashmir Shaivism

This commentator reads the verse from the standpoint of one who sees all as the same. For such a person the very division into enemy, friend, and the rest is a mere outward dealing, a surface transaction, not anything inward, because he is settled in Brahman. The steadiness is the natural way of being for one whose inner ground is undivided.

Ācārya Abhinavagupta

Bhakti

These commentators read the verse as the lakshana, the defining mark, of one who has attained Brahman, and several spell out its logic as a single causal chain run backwards: the absence of delusion (moha) begets the steady, unwavering understanding, which in turn begets the absence of elation and despondency. They stress that exultation and sorrow are themselves bound up with the false conceit of self and are therefore mere delusion. One reading offers a vivid image: such a person is no more moved by good or bad events than a mountain can be swept away by a flood of mirage, and so is even-tempered, the very Supreme Brahman in living form.

Śrīdhara Svāmī · Śrīla Viśvanātha · Śrīla Baladeva · Sant Jñāneśvar

Modern

These commentators restate the verse in plain terms as the state of the liberated sage who keeps a balanced mind and is never deluded, having abandoned all action and resting in Brahman, while one who identifies with body and mind feels exhilaration at a pleasant object and grief at an unpleasant one. One reading develops a careful psychological point: merely knowing that something pleasant or unpleasant has come is no fault; the fault lies only in the inner instrument (antahkarana) being affected, in the rising of joy and grief. This happens when the ego, taking the actions of nature as its own, holds 'I am the doer'; but the one whose delusion is gone, knowing 'the qualities move only among the qualities,' experiences his real non-doership, and for the true Self being elated or agitated is simply not possible. This reading also presses the last words further: when Brahman is truly experienced, no second person remains who is 'settled in Brahman'; so long as anyone still takes himself to be settled in Brahman, there is still a limitation, a falling-short, in the real experience.

Swami Sivananda · Lokmanya Tilak · Swami Ramsukhdas