Key Arguments

• He begins by restricting the causes of movement to desire and understanding (with imagination counted as a kind of understanding): "But apparently at any rate these two things do cause movement, either desire or understanding—if we posit the imagination as a sort of understanding. 417".
• He notes that imagination can override scientific knowledge and that non‑rational animals have imagination but not understanding or rational calculation: "For many people | 433 a 10 | follow their imaginations against their scientific knowledge, and in the other animals understanding and rational calculation do not exist, although imagination does. 418". This shows that imagination (and so desire linked to it) can independently move animals.
• He explicitly identifies understanding and desire as the two faculties capable of causing locomotion, and specifies that the relevant understanding is practical, not theoretical: "Both of these, therefore, are capable of causing movement with respect to place, namely, understanding and desire—understanding, {61} however, that is of the practical sort, which rationally calculates for the sake of something, and differs from the theoretical sort in respect of the end. 419".
• He makes desire essentially teleological—always 'for the sake of something'—and ties it directly to the starting‑point of practical understanding and to action: "And every desire too is for the sake of something. | 433 a 15 | For the object of desire is the starting-point of practical understanding, and the last thing is the starting-point of the action. 420".
• He explains why desire and practical thought appear as causes: "So it is reasonable that these two things—desire and practical thought—appear to be the causes of movement. For the object of desire moves us, and, because of this, thought causes movement, because its starting-point is the object of desire." Thought is derivative: it moves only because it takes as its starting‑point the desired object which already moves.
• He makes movement by imagination explicitly conditional on desire: "Also, when the imagination causes movement it does not cause movement in the absence of desire. | 433 a 20 |". So imagination alone is insufficient; desire is required.
• He then compresses the result into a single principle: "There is, then, one thing that causes movement, namely, the desiring part." Even though understanding also seems to move, he argues they would need a common form if two distinct movers existed: "For if there were two things that caused movement, understanding and desire, they would cause movement in virtue of some common form. 421".
• He shows that understanding does not move without desire because wish is a kind of desire and rationally calculated movement is in accordance with wish: "But as things stand the understanding evidently does not cause movement without desire (for wish is a desire, and when something is moved in accord with rational calculation, it is moved in accord with wish)".
• He shows that desire can move even against correct calculation, since appetite is a type of desire: "and desire causes movement even contrary to rational calculation, since appetite | 433 a 25 | is a sort of desire." This asymmetry (desire can move without, and even against, intellect; intellect cannot move without desire) supports his claim that desire is the mover in form.
• He contrasts the correctness of understanding with the fallibility of desire and imagination to mark desire (and its objects) as the true source of motivational error: "Now understanding is always correct, whereas desire and imagination are both correct and incorrect. 422".
• He concludes that what moves in every case is the object of desire conceived as the good or apparent good within the scope of action: "That is why what causes movement in every case is the object of desire, which is either the good or the apparent good—not every good, however, but the good that is doable in action. 423". This restricts the mover to practical, contingent goods.
• He further characterizes the practical domain as what can be otherwise, limiting the mover to contingent matters: "And what is doable in action is what admits of being otherwise. 424 | 433 a 30 |".
• From these points he infers that the moving capacity is precisely desire: "Evidently, then, it is a capacity of the soul of this sort—the one called desire—that causes movement."

Source Quotes

III 10 But apparently at any rate these two things do cause movement, either desire or understanding—if we posit the imagination as a sort of understanding. 417 For many people | 433 a 10 | follow their imaginations against their scientific knowledge, and in the other animals understanding and rational calculation do not exist, although imagination does. 418 Both of these, therefore, are capable of causing movement with respect to place, namely, understanding and desire—understanding, {61} however, that is of the practical sort, which rationally calculates for the sake of something, and differs from the theoretical sort in respect of the end.

III 10 But apparently at any rate these two things do cause movement, either desire or understanding—if we posit the imagination as a sort of understanding. 417 For many people | 433 a 10 | follow their imaginations against their scientific knowledge, and in the other animals understanding and rational calculation do not exist, although imagination does. 418 Both of these, therefore, are capable of causing movement with respect to place, namely, understanding and desire—understanding, {61} however, that is of the practical sort, which rationally calculates for the sake of something, and differs from the theoretical sort in respect of the end. 419 And every desire too is for the sake of something. | 433 a 15 | For the object of desire is the starting-point of practical understanding, and the last thing is the starting-point of the action.

417 For many people | 433 a 10 | follow their imaginations against their scientific knowledge, and in the other animals understanding and rational calculation do not exist, although imagination does. 418 Both of these, therefore, are capable of causing movement with respect to place, namely, understanding and desire—understanding, {61} however, that is of the practical sort, which rationally calculates for the sake of something, and differs from the theoretical sort in respect of the end. 419 And every desire too is for the sake of something. | 433 a 15 | For the object of desire is the starting-point of practical understanding, and the last thing is the starting-point of the action. 420 So it is reasonable that these two things—desire and practical thought—appear to be the causes of movement.

418 Both of these, therefore, are capable of causing movement with respect to place, namely, understanding and desire—understanding, {61} however, that is of the practical sort, which rationally calculates for the sake of something, and differs from the theoretical sort in respect of the end. 419 And every desire too is for the sake of something. | 433 a 15 | For the object of desire is the starting-point of practical understanding, and the last thing is the starting-point of the action. 420 So it is reasonable that these two things—desire and practical thought—appear to be the causes of movement. For the object of desire moves us, and, because of this, thought causes movement, because its starting-point is the object of desire.

420 So it is reasonable that these two things—desire and practical thought—appear to be the causes of movement. For the object of desire moves us, and, because of this, thought causes movement, because its starting-point is the object of desire. Also, when the imagination causes movement it does not cause movement in the absence of desire. | 433 a 20 | There is, then, one thing that causes movement, namely, the desiring part.

For the object of desire moves us, and, because of this, thought causes movement, because its starting-point is the object of desire. Also, when the imagination causes movement it does not cause movement in the absence of desire. | 433 a 20 | There is, then, one thing that causes movement, namely, the desiring part. For if there were two things that caused movement, understanding and desire, they would cause movement in virtue of some common form.

For if there were two things that caused movement, understanding and desire, they would cause movement in virtue of some common form. 421 But as things stand the understanding evidently does not cause movement without desire (for wish is a desire, and when something is moved in accord with rational calculation, it is moved in accord with wish), and desire causes movement even contrary to rational calculation, since appetite | 433 a 25 | is a sort of desire. Now understanding is always correct, whereas desire and imagination are both correct and incorrect.

421 But as things stand the understanding evidently does not cause movement without desire (for wish is a desire, and when something is moved in accord with rational calculation, it is moved in accord with wish), and desire causes movement even contrary to rational calculation, since appetite | 433 a 25 | is a sort of desire. Now understanding is always correct, whereas desire and imagination are both correct and incorrect. 422 That is why what causes movement in every case is the object of desire, which is either the good or the apparent good—not every good, however, but the good that is doable in action. 423 And what is doable in action is what admits of being otherwise.

Now understanding is always correct, whereas desire and imagination are both correct and incorrect. 422 That is why what causes movement in every case is the object of desire, which is either the good or the apparent good—not every good, however, but the good that is doable in action. 423 And what is doable in action is what admits of being otherwise. 424 | 433 a 30 | Evidently, then, it is a capacity of the soul of this sort—the one called desire—that causes movement.

422 That is why what causes movement in every case is the object of desire, which is either the good or the apparent good—not every good, however, but the good that is doable in action. 423 And what is doable in action is what admits of being otherwise. 424 | 433 a 30 | Evidently, then, it is a capacity of the soul of this sort—the one called desire—that causes movement. For those, on the other hand, who divide the soul into parts, | 433 b 1 | and divide and separate them in accord with their capacities, the parts turn out to be very many—the nutritive part, perceptual part, understanding part, deliberative part, and further the desiring part.

423 And what is doable in action is what admits of being otherwise. 424 | 433 a 30 | Evidently, then, it is a capacity of the soul of this sort—the one called desire—that causes movement. For those, on the other hand, who divide the soul into parts, | 433 b 1 | and divide and separate them in accord with their capacities, the parts turn out to be very many—the nutritive part, perceptual part, understanding part, deliberative part, and further the desiring part.

Key Concepts

• But apparently at any rate these two things do cause movement, either desire or understanding—if we posit the imagination as a sort of understanding. 417
• For many people | 433 a 10 | follow their imaginations against their scientific knowledge, and in the other animals understanding and rational calculation do not exist, although imagination does. 418
• Both of these, therefore, are capable of causing movement with respect to place, namely, understanding and desire—understanding, {61} however, that is of the practical sort, which rationally calculates for the sake of something, and differs from the theoretical sort in respect of the end. 419
• And every desire too is for the sake of something. | 433 a 15 | For the object of desire is the starting-point of practical understanding, and the last thing is the starting-point of the action. 420
• For the object of desire moves us, and, because of this, thought causes movement, because its starting-point is the object of desire.
• Also, when the imagination causes movement it does not cause movement in the absence of desire. | 433 a 20 |
• There is, then, one thing that causes movement, namely, the desiring part.
• the understanding evidently does not cause movement without desire (for wish is a desire, and when something is moved in accord with rational calculation, it is moved in accord with wish), and desire causes movement even contrary to rational calculation, since appetite | 433 a 25 | is a sort of desire.
• Now understanding is always correct, whereas desire and imagination are both correct and incorrect. 422
• what causes movement in every case is the object of desire, which is either the good or the apparent good—not every good, however, but the good that is doable in action. 423
• what is doable in action is what admits of being otherwise. 424 | 433 a 30 |
• Evidently, then, it is a capacity of the soul of this sort—the one called desire—that causes movement.

Context

Early to middle of III.10 (433a10–433a30), where Aristotle synthesizes his earlier exclusions (of nutritive, perceptual, and purely theoretical understanding as movers) into a positive account: desire and practical intellect are the proximate causes of animal locomotion, but intellect moves only through desire, so that the desiring part, oriented to the good or apparent good that is doable in action, is the formal principle of self‑movement.