The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut [1896] at sacred-texts.com
20. Since (the highest Brahman) is inside (of the limiting adjuncts), it participates in their increase and decrease; owing to the appropriateness (thus resulting) of the two (things compared) it is thus (i.e. the comparison holds good).
The parallel instance (of the sun's reflection in the water) is unobjectionable, since a common feature--with reference to which alone the comparison is instituted--does exist.
[paragraph continues] Whenever two things are compared, they are so only with reference to some particular point they have in common. Entire equality of the two can never be demonstrated; indeed if it could be demonstrated there would be an end of that particular relation which gives rise to the comparison. Nor does the sûtrakâra institute the comparison objected to on his own account; he merely sets forth the purport of a comparison actually met with in scripture.--Now, the special feature on which the comparison rests is 'the participation in increase and decrease.' The reflected image of the sun dilates when the surface of the water expands; it contracts when the water shrinks; it trembles when the water is agitated; it divides itself when the water is divided. It thus participates in all the attributes and conditions of the water; while the real sun remains all the time the same.--Similarly Brahman, although in reality uniform and never changing, participates as it were in the attributes and states of the body and the other limiting adjuncts within which it abides; it grows with them as it were, decreases with them as it were, and so on. As thus the two things compared possess certain common features no objection can be made to the comparison.