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

19. But there is no parallelism (of the two things compared), since (in the case of Brahman) there is not apprehended (any separate substance) comparable to the water.

Since no substance comparable to the water is apprehended in the case of Brahman, a parallelism between Brahman and the reflected images of the sun cannot be established. In the case of the sun and other material luminous bodies, there exists a separate material substance occupying a different place, viz. water; hence the light of the sun, &c., may be reflected. The Self, on the other hand, is not a material thing, and, as it is present everywhere and all is identical with it, there are no limiting adjuncts different from it and occupying a different place.--Therefore the instances are not parallel.

The next Sûtra disposes of this objection.