The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut [1890] at sacred-texts.com

24. And on account of the statement of reflection (on the part of the Self).

The fact of the sacred texts declaring that the Self reflected likewise shows that it is the operative as well as the material cause. Passages like 'He wished, may I be many, may I grow forth,' and 'He thought, may I be many, may I grow forth,' show, in the first place, that the Self is the agent in the independent activity which is preceded by the Self's reflection; and, in the second place, that it is the material cause also, since the words 'May I be many' intimate that the reflective desire of multiplying itself has the inward Self for its object.