The intellect [is] the cause of motion according to place among human beings, for it sets in motion the living being. By contrast, sensation [is the cause of motion] among irrational things. Alongside these there is another factor which sets in motion according to place; for the desiderative faculty [is] a cause of motion according to place. But this is different: for desire is present in the rational faculties, and among the irrational [faculties].
Motion [sc. is of the following kinds]: in accordance with place, in accordance with alteration, in accordance with growth, in accordance with decay.
Motion [sc. takes six forms]: 1, growth; 2, diminution; 3, locomotion; 4, alteration; 5, generation; 6, corruption.
To be moved is also to change from place to place. The things that move in a circle [are] incorruptible.
[We can speak] of motion in another way. Motion: it is not possible, he says, for a lump to be moved in accordance with nature downwards, nor for fire to be displaced upwards; for these things do not have the principle of such a motion in themselves; rather, [they are] set in motion by another thing from without.[1] For each of the elements is present in its own wholeness: for the totalities wish to be present or to be moved in a circle, because the circular motion is also a certain rest. And by nature the lump which is moved [is] immovable by its own wholeness; and this [is also the case with] fire, because of its own sphere. But when in the domain of what is contrary to nature a lump or water or this air is produced, it is driven on toward the wholeness of each thing itself, by seeking to recover its appropriate rest. For when they are displaced by the action of some external force from that which is in accordance with nature, they are moved on the course toward what is contrary to nature. They are moved in [their] course, since they are in an alien place and are deprived of their peculiar wholeness contrary to nature. So the motion that is directed toward the natural place is not in accordance with nature, for, if it were, the totalities would also set in motion these motions. Consequently, such movements of the parts are so not in accordance with nature but the distances traveled are directed toward what is in accordance with nature. However, the expression "in accordance with nature" can be understood in another way: just as restoration to health, too, is said to be in accordance with nature, so falling sick [is said to be] contrary to nature, since the former leads toward what is in accordance with nature and the latter [leads toward] what is contrary to nature. For the primary mover, if it is a body, is moved as well. For not only does the stick move the door but the hand [moves] the stick, not itself remaining unmovable but being moved.[2] But if the primary mover is incorporeal, there is no necessity that it is moved when moving something else,[3] since every divine mover is unmovable as well, in having a standing substance and power and activity. [To put it] another way: no mover amongst the objects of desire is [itself] moved; for instance, beauty is not moved, despite frequently setting in motion the lover;[4] nor is the image, despite moving the one who is looking at it; etc. Plato, when calling the soul 'self-moved',[5] is not speaking of motion in accordance with place. The soul often changes from what [is in] potentiality to what [is in] activity, since it changes from not knowing this to knowing [it].[6] But if it changes, it is moved too. Such a motion and change [is] a certain generation -- but not simply a generation of substance. Motion differs from activity. For motion is an incomplete activity, but activity is incessant;[7] that is why the divine things are also activities without potentiality. But it is a matter for investigation, whether the motions in the mover and in the moved are two or a single one, and of which there would be motion, either of the mover or of the moved. And they maintain that [the motion is] of the moved: for he is the one advancing from the incomplete to the complete, not the mover, just as it happens in the case of the pupil and the teacher;[8] for the motion in both of them [is] one, begun by the teacher but completing in the pupil. Consequently the soul, too, is moved; for it changes from the incomplete to the complete and it does not have an incessant activity, and it is not without potentiality either. [To put it] another way: in those things [in which there is] the prior and the posterior, [there is] time too; and in the things [in which there is] time, [there is] motion too; for these also change over; hence in those things [in which there is] motion, [there is] time too. In the soul there is the prior and the posterior; thus motion [too]. In the soul [there is] the prior and the posterior; at any rate they proceed from premise to conclusions, and [the soul] does not know everything at the same time, but one thing before another. Nor does it undertake at the same time everything that it knows, but the one first and the other afterwards, and in general proceeds from virtue to vice, and from ignorance to knowledge. So Plato did well when he assigned self-motion to it [sc. the soul].[9] And Aristotle, having called the soul unmovable, did not say that this motion is not attached to the soul but that for us there are no physical and acknowledged motions attached to it. For it neither grows nor diminishes, and it changes neither in accordance with quality nor in accordance with quantity; instead, in accordance with some other intelligible and, especially, living motions. The soul is moved in accordance with alteration, in that it changes from a disposition into a state, [for instance] from ignorance into knowledge. Aristotle declared that [the soul] is not moved, since he was focusing on the bodily motions. For if the soul were moved not incidentally, there would be a place for it too; for all motions [are] in a place, [so] obviously the bodily [ones are too]; for everything in a place is a body.
Of the things subject to motion, some have their motion in accordance with substance, others [are subject to motion] incidentally.
Of the things [that are subject to motion] incidentally: some are moved by virtue of having a motion of their own, such as a sailor; for he is incidentally moved on the ship which is being moved, and he also is able to move himself; others are incidentally moved by virtue of being unmovable by themselves, such as the rowers.
Of the things subject to being moved by themselves, that is to say in accordance with substance: some have a motion joined essentially and, because of this, able to complete the substance, such as the heavenly bodies; others do not have a motion joined essentially or able to complete the substance, but they take part in the motion when the mover is present in them, such as the lump and the other separated parts of the elements.