Supreme happiness is inward and steady.

Krishna continues describing the state of union by pointing to the kind of joy it holds. This is sukham atyantikam — infinite, absolute bliss — utterly unlike the fleeting pleasures of the world. Two qualities distinguish it: it is buddhi-grahyam, apprehended by the refined intellect rather than the senses, and atindriyam, entirely beyond sensory experience. The senses, which deliver all worldly delight, cannot touch this joy at all.

This matters profoundly. Every sense-pleasure depends on an outer object, fades with contact, and leaves a residue of craving. The bliss of the Self depends on nothing, fades for nothing, and leaves no lack.

Once a person tastes it and becomes sthita — established in it — he 'never moves from the Reality'. He has found the unshakable ground. Worldly joys could always be lost; this one cannot, because it is not given by anything outside and so cannot be taken away.