Śaṅkaraon Brahma Sūtra 2.3.39

Moreover the meditation taught in the Vediinta-texts, whose aim is the realisation of the Self as rcprCsSented by the Upanishads, is possible only if the Self is the agent Compare the following passages, ‘Verily, the Self is to be seen, to be heard, to be perceived, to be marked ' {Bri, Up. ri, 4, 5); ‘The Self we must seek out, we must try to understand’ (A'A. Up. VIII, 7, i); ‘Meditate on the Self as Om’ (Mu. Up. II, 2, 6).—Therefrom also it follows that the Self is an agent.