The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
38. On account of the reversal of power.
The soul distinct from 'understanding' has to be viewed as an agent for the following reason also. If the buddhi which is denoted by the term 'understanding' were the agent, there would take place a reversal of power, i.e. the instrumental power which appertains to the buddhi would have to be set aside, and to be replaced by the power of an agent. But if the buddhi has the power of an agent, it must be admitted that it is also the object of self-consciousness
[paragraph continues] (ahampratyaya) 1, since we see that everywhere activity is preceded by self-consciousness, 'I go, I come, I eat, I drink,' &c. But if the buddhi is endowed with the power of an agent and effects all things, we have to assume for it another instrument by means of which it effects everything. For we see that agents although themselves capable of acting yet become really active only through making use of instruments.--Hence the whole dispute is about a name only, and there is no real difference, since in either case that which is different from the instrument of action is admitted to be the agent.
52:1 And that would virtually identify the buddhi with the gîva, the individual soul.