It seems at any rate that these two are responsible for movement: either desire or intellect, if someone would consider imagination as a kind of thinking; for many people follow their imaginations in spite of their knowledge, and beside this in all other animals neither thinking nor reasoning reside, but imagination does. Both these, therefore, are responsible for movement in respect of place, namely intellect and desire, yet practical intellect, and the one which reasons for the sake of something; for it differs from contemplative intellect in respect of the end. But every desire is also for the sake of something; for that to which desire refers is the beginning of the practical intellect, and the point whereat our thought comes is the beginning of the action. So it is with reason that these two seem to be responsible for movement, desire and practical thought; for the object of desire causes movement, and because of this thought causes movement too, in that it is its start. But also when imagination starts a movement it does not do so without desire. So what produces movement is numerically one, i.e. the object of desire. For if the producers were two, intellect and desire, they would produce movement in virtue of some common form (some common faculty). But as it is, the intellect does not seem to start a movement without desire (for wish is a kind of desire), and when it is moved in accordance with the reasoning, it is also moved in accordance with the wish, while desire produces movement against reasoning — for wish is a kind of desire. Intellect, then, is always right; whereas desire and imagination are both right and not right. That is why the object of desire always produces movement, yet this is either the good or what seems to be so. Not every good but that which is concerned with actions. And such is whatever may be otherwise (that is, whatever may both come to be and not come to be).
Bibliography: Aristotle De anima (433a.9 to 433a.30)
Translation: George Kotsalis