Volume 17, Issue 3, 1969
Aristotle’s Idea of Motion
Motion, which St Thomas Aquinas made the starting point of his "first way”, is so universal and obvious that when Aristotle comes to discuss it in his two books of Metaphysics, and even more in his six books of Physics, he does not attempt to prove its reality. But, bearing in mind what Parmenides, Zeno of Elea, Heraclite s, and Plato in particular had told of motion, he tries to determine it with a strict definition, then to divide it into genera, among which he puts the local movement to the foreground. That is why, the present article, dealing with the Stagirite’s position in that regard, is divided in three essential parts.
In part I the author attempts to show the difference — as seen by Aristotle — between "change” and "motion”, then to submit to thorough analysis the only Aristotelian definition of motion as "act” of being existent in potentiality, insofar as it is in potentiality, (III Phys., 1,201 a 10 ff.). There he remarks that the subject of motion belongs to the essence thereof so that the act cannot be as much as thought of without an object. It is, however, an unfinished act, remaining, as regards its destination point, in the relation of potentiality to actuality. From the definition of motion derive some of its properties, first of all that of its sequential continuity, enclosed between two extremes (a quo and ad quem), in the relation of contrariness to one another. This implies that motion must take place in time.
In part II it is emphasised that movement, thus defined, divides-according to Aristotle, in three basic genera: motion belonging to the category of quantity (increase and loss), to that of quality (arising and destruction of accidental sensuous qualities), and that of location (local movement). In doing so, the author shows that what Aristotle calls "arising” and "destruction” in his Phys. III ch. 1, is not substantive arising and destruction, but those of accidental physical qualities, the so-called ’’varieties” in the general acceptation of the word. On the other hand, the ’’variety” referred to by Aristotle earlier in the same text, is variety in the narrow sense (i. e. augmentation or diminution of a type of motion). Variety is, in fact, described as motion not only as of something which is capable of changing, but "insofar as it is variable”. We then have here a fresh attempt at eliminating the apparent contradiction between the definition of motion and what Aristotle says of its genera in Bk. III, c. 1, and next between these and his statements on the essence and genera in Bk. V and the following.
Part III, using the Stagirite’s own words- shows the primacy of local motion on quantitative as well as qualitative motion, primacy regarding nature, time and perfection. Consequently, and on the basis of contemporary natural science, Aristotle comes to the acceptance of one continuous movement which, being eternal, requires the existence of an eternal first mover, Motor, who, while remaining immobile itself, sets the whole world into motion.