Śaṅkaraon Brahma Sūtra 2.3.32

The internal organ which constitutes the limiting ad- junct of the soul is called in different places by different names, such as manas (mind), buddhi (intelligence), vi^wAna (knowledge), ^itta (thought). This difference of nomen- clature 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 per- petual perception or perpetual non-perception. There would result perpetual perception whenever there is a con- junction 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 6f the Self or the sense-organs. But the former is not possiblfc, as the 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 mo- ment 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 ' (Brf. Up. I, 5, 3). Scripture moreover shows that desire and similar states are modifications of the mind, ‘ Desire, repre- sentation, doubt, faith, want of faith, memory, forgetfulness, shame, reflection, fear, all this is mind.* The explanation given in Sutra 29 is therefore an appropriate one.