The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
32. Otherwise (if no manas existed) there would result either constant perception or constant non-perception, or else a limitation of either of the two (i.e. of the soul or of the senses).
The internal organ which constitutes the limiting adjunct of the soul is called in different places by different names, such as manas (mind), buddhi (intelligence), vigñâna (knowledge), kitta (thought). This difference of nomenclature is sometimes made dependent on the difference of the modifications of the internal organ which is called manas when it is in the state of doubt, &c., buddhi when it is in the state of determination and the like.--Now we must necessarily acknowledge the existence of such an internal organ; because otherwise there would result either perpetual perception or perpetual non-perception. There would result perpetual perception whenever there is a conjunction of the soul, the senses and the objects of sense--the three together constituting the instruments of perception; or else, if on the conjunction of the three causes the effect did not follow, there would take place perpetual non-perception. But neither of these two alternatives is actually observed.--Or else we should have to assume that there are obstacles in the way of the energy either of the Self or the sense-organs. But the former is not possible, as the
[paragraph continues] Self is not capable of any modification; nor the latter, as we cannot assume that the energy of the sense-organ which is non-obstructed in the preceding and the following moment should, without any cause, be obstructed (in the intervening moment). Hence we have to acknowledge the existence of an internal organ through whose attention and non-attention perception and non-perception take place. Thus scripture declares, 'My mind was elsewhere, I did not see; my mind was elsewhere, I did not hear; for a man sees with his mind and hears with his mind' (Bri. Up. I, 5, 3). Scripture moreover shows that desire and similar states are modifications of the mind, 'Desire, representation, doubt, faith, want of faith, memory, forgetfulness, shame, reflection, fear, all this is mind.' The explanation given in Sûtra 29 is therefore an appropriate one.