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

26. (Brahman is the material cause) on account of (the Self) making itself; (which is possible) owing to modification.

Brahman is the material cause for that reason also that Scripture--in the passage, 'That made itself its Self' (Taitt. Up. II, 7)--represents the Self as the object of action as well as the agent.--But how can the Self which as agent was in full existence previously to the action be made out to be at the same time that which is effected by the action?--Owing to modification, we reply. The Self, although in full existence previously to the action, modifies itself into something special, viz. the Self of the effect. Thus we see that causal substances, such as clay and the like, are, by undergoing the process of modification, changed into their products.--The word 'itself' in the passage quoted intimates the absence of any other operative cause but the Self.

The word 'parinâmât' (in the Sûtra) may also be taken as constituting a separate Sûtra by itself, the sense of which would be: Brahman is the material cause of the world for that reason also, that the sacred text speaks of Brahman and its modification into the Self of its effect as co-ordinated, viz. in the passage, 'It became sat and tyat, defined and undefined' (Taitt. Up. II, 6).