The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut [1896] at sacred-texts.com

28. Or else like that of light to its substratum, both being fire.

Or else the relation of the two may be viewed as follows. Just as the light of the sun and its substratum, i.e. the sun himself, are not absolutely different--for they both consist of fire--and yet are spoken of as different, so also the soul and the highest Self.