The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
4. And because there is a (separate) denotation of the object of activity and of the agent.
The attributes of consisting of mind, and so on, cannot belong to the embodied Self for that reason also, that there is a (separate) denotation of the object of activity and of the agent. In the passage, 'When I shall have departed from hence I shall obtain him' (Kh. Up. III, 14, 4), the word 'him' refers to that which is the topic of discussion, viz. the Self which is to be meditated upon as possessing the attributes of consisting of mind, &c., as the object of an activity, viz. as something to be obtained; while the words, 'I shall obtain,' represent the meditating individual Self as the agent, i.e. the obtainer. Now, wherever it can be helped, we must not assume that one and the same being is spoken of as the agent and the object of the activity at the same time. The relation existing between a person meditating and the thing meditated upon requires, moreover, different abodes.--And thus for the above reason, also, that which is characterised by the attributes of consisting of mind, and so on, cannot be the individual soul.