SBE 48: The Vedanta-Sutras with commentary by Ramanuja, G...
35. And on account of the designation (of the Self as the agent) in actions. If not so, there would be change of grammatical expression.
Because in the text 'Knowledge performs the sacrifice, it performs all works' (Taitt. Up. II, 5) the Self is designated as the agent in all worldly and Vedic works, for this reason also the Self must be held to be an agent. And should it be said that the word 'knowledge' in that text denotes not the Self, but the internal organ or buddhi, we point out that in that case there would be a change of grammatical expression, that is to say, as the buddhi is the instrument of action, the text would exhibit the instrumental case instead of the nominative case 'by knowledge, and so on' (vigñânena instead of vigñânam).