The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
34. And on account of (the text) teaching its wandering about.
The quality of being an agent has to be attributed to the soul for that reason also, that, in a chapter treating of the soul, the text declares it to wander about in the state of sleep, 'The immortal one goes wherever he likes' (Bri. Up. IV, 3, 12);
and again, 'He moves about, according to his pleasure, within his own body' (Bri. Up. II, 1, 18).