Compendium on the Soul

The motion of celestial bodies is most necessary and continuous; it is the cause of generation and corruption original: "generōnis, & corruptiōis"; the Aristotelian concept of things coming into being and passing away., precedes other motions, and is presupposed by them, as is evident in the book On the Heavens. Therefore, if all bodies agreed in a similar motion, it would follow that they agree in a necessary and continuous motion. If this were so, then nothing would be found to be at rest, because no body could be found resting in any way, nor would any body be found to move in a straight motion original: "motu recto"; linear motion, which in ancient physics was characteristic of the Four Elements (earth, water, air, fire) moving to their natural place.. This straight motion, by its very definition, is said to proceed and terminate from rest to rest. We must also consider that, besides natural straight motion, there is the voluntary motion of animals toward various directions, which also begins and ends in rest.

Therefore, if all bodies did not differ by diverse motions—namely, through the fact that some move with circular motion and some with natural straight motion, or with the aforementioned voluntary motion—then all bodies would agree in circular motion. Thus, no thing would be found having rest, nor having straight motion; indeed, nothing would be found except circular motion. That straight motion would not be found is clear from its definition: that it proceeds from and terminates in rest. From this it is clear that it is distinct from continuous circular motion. For if all bodies did not differ by the diverse motions mentioned above, it follows that all bodies would move with a similar motion. Thus follows the conclusion of Avicenna Ibn Sina (c. 980–1037), the Persian polymath whose work on the soul integrated Aristotelian and Neoplatonic thought.: namely, that if all bodies which share in the fact that they are "body" did not differ in the fact that they move with diverse motions, then nothing would be found at rest, nor indeed would anything be found except circular motion.

Chapter Two: On the first division of the powers of the soul, and on the absolute definition of the soul.

We have already established the explanation of how, whenever things share in something and differ in another, that in which they share is distinct from that in which they differ. Subsequently, we find composite animate bodies—that is, those possessing a soul—already sharing and differing in two properties: their motion and their apprehension original: "apprȩhensione"; the ability of a living being to perceive or grasp information from its environment.. In motion, certainly, all these things share in the fact that they move with quantitative motion—that is, the motion of growth original: "motu augumentationis"; increase in size or development.. They differ, however, in that some of them move, in addition to this, with local motions according to their will, while some of them do not move in this way, such as plants. In a similar way, the bodies of animals share in the fact that they are sensitive and apprehensive by some species of sensory apprehension; they then differ, because some of them apprehend...