The bright and the dark: the bright, because it makes knowledge shine; the dark, for want of that. These two are held to be the two courses of the world, that is, of those eligible for knowledge and for action, not of the whole world; they are everlasting, eternal, since transmigration is eternal. By the one, the bright, one goes to non-return; by the other one returns again, comes back.
Contemporary English rendering of the Sanskrit bhāṣya, pending scholar review.