For Aristotle, and the medievals in his wake, motion was not merely a starting point from everyday experience, but a phenomenon whose nature needed closer investigation. The question about the nature of motion not only concerned the adequacy of Aristotle’s definition of motion, the quid nominis so to speak, but also, more interestingly, the question of what motion really is, that is, the quid rei or ontological status of motion. In response to certain conceptual puzzles that the Eleatic philosophers Parmenides (fl. 480 BCE) and Zeno (fl. 450 BCE) had raised, Aristotle had introduced his form-matter theory of motion. If we may believe Aristotle’s account, Parmenides and Zeno had claimed that none of the things that exist come into being or pass away, or, in other words, that change is only apparent. They had argued that what comes to be must either do so from what already is, in which case it is no veritable coming-to-be, or from nothing at all (ex nihilo). The latter option, however, was considered absurd. On these logical grounds, they denied that change was possible.
Aristotle starts from the common sense assumption that perceived change is real. By his doctrine of form and matter, Aristotle tries to solve the logical impasse created by Parmenides and Zeno. He considers the objects in the world as composites of underlying matter and imposed form. From the perspective of matter, change involves continuation. The underlying substrate does not change. From the perspective of form, however, change involves real change, because it consists of the successive replacement of one form by another. For example, if a black object turns white, the matter or substrate remains, whereas the form of blackness is replaced by that of whiteness. In similar fashion, if an acorn becomes an oak tree, the change can be described in terms of the continuation of a substrate on which a new form is imposed.
In Aristotle’s view, the replacement of one form by another is not a transition from nonbeing to being. He supposed that any change was a transition from potentiality to actuality. The black, or rather, the not-white, is potentially white. By becoming white, it becomes actually what it was already potentially. Similarly, an acorn is potentially an oak. In sum, Aristotle’s invocation ofmatter and form, and of potentiality and actuality solved the logical puzzles raised by the Eleatic philosophers. Change does not involve a passage from nonbeing to being, which both Aristotle and Parmenides considered impossible, but rather a passage from potential being to actual being. As a consequence, matter and form, and potentiality and actuality became the most fundamental explanatory principles of medieval physics. Form is the principle that bears the essential properties of a substance, that is, of any object in reality. It determines what the thing is, that is, what its specific nature is. Matter is the passive recipient of the form (see the entry on Form and Matter in this volume). Medieval attempts to define motion and discuss its ontological status were later ridiculed by Descartes. In the Rules for the Direction of the Mind (Regulae ad directionem ingenii, Rule 12 AT X, 427-27), Descartes pokes fun at the Aristotelian definition of motion. ‘‘Who doesn’t know what motion is?,’’ he asks rhetorically. In The World (Le Monde, AT XI 39), started around the same time, Descartes even claims that he finds the scholastic definition of motion so obscure that he is forced to leave it in ‘‘their language,’’ that is, ‘‘motion is the actuality of a thing in potentiality insofar as it is in potentiality’’ (motus est actus entis in potentia prout in potentia est).
In his works, Aristotle had made contradictory statements concerning the ontological status of motion. In Physics, book 3 (200b32-201a10) he maintained that motion is not something over and above the things in motion. In other words, motion does not constitute a separate category, but belongs to the same category that is gained by motion, that is, the category of Place in the case of local motion. In the Categories (11b1-8), however, Aristotle had claimed that motion fell into the category of Affection (passio).
Averroes tried to reconcile these incompatible statements by pointing out that in the Physics, Aristotle had set forth the more correct view, whereas in the Categories, he had maintained the more common view. Averroes’ explanation of Aristotle’s view hinges on an analysis of motion from two different perspectives. Motion, if considered from the terminus toward which it tends, only differs from it in its degree of ‘‘more or less,’’that is, in its degree of perfection. If, however, motion is considered as a process (via) toward perfection or actuality, and, as a consequence, is different from the perfection it attains, it belongs to a category of its own. When seen as a road Toward actuality, motion cannot coincide with that actuality. The same twofold analysis of motion recurs in Averroes’ commentary on Physics V. There it is couched in the terminology of change ‘‘according to matter’’ and ‘‘according to form.’’ According to matter, change and its terminus belong to the same category; according to its form, one must view change as a transmutation that takes place in time and constitutes a category of its own, namely that of Affection (passio).
In the thirteenth and fourteenth centuries, these alternative analyses of motion came to be captured under the formulas forma fluens and fluxusformae, a distinction that medieval authors usually attributed to Albert the Great. According to the forma fluens theory, change is nothing but the forms successively gained by the changeable body. In the case of local motion, the forma fluens is the place successively attained by the mobile body. In other words, motion is the same as the perfection or form it acquires, but it represents that form in a state of flux. It is important to note that the flowing character of the form is not posited in the form itself, but results from the degree of actualization of the form in the subject. Thus, the view of motion as forma fluens did not contradict the common medieval view that forms are unchangeable. The fluxus formae theory, on the other hand, maintained that change is not the form acquired but is ‘‘the flux’’ of that form - the flow, the process, or the road toward an actuality or perfection. These distinctions were, at least implicitly, in the background of fourteenth-century discussions of Aristotle’s statement that there is no change over and above real things.
In the fourteenth century, the two main positions in the debate over the nature of motion were clear: some claimed that motion is a flux, which is distinct from the mobile object and the place, whereas others advocated that motion requires nothing more than mobile body and place. However, the debate was complicated by other elements that were woven into the discussion, such as the correct interpretation of Aristotle’s and Averroes’ views and a discussion of whether local motion should be treated in the same way as alteration, that is, change of quality. On the basis of arguments that invoke God’s omnipotence in rotating the whole cosmos, John Buridan had argued that local motion cannot be treated in the same way as alteration. Albert of Saxony found Buridan’s arguments concerning God’s omnipotence compelling and gave up the forma fluens theory for local motion, which in his view represented the theory genuinely advocated by Aristotle and Averroes.
John Buridan, Nicholas Oresme, and Albert of Saxony describe local motion as fluxus and as ‘‘being continuously in another way than before’’ (aliter et aliter se habere quam prius). But what is fluxus? Buridan and Albert of Saxony agree that the fluxus character of motion should be interpreted as an inherent quality or disposition (dispositio) of the mobile object, as a property of a mobile being, but of such a nature that it is purely successive.
Oresme, on the other hand, rejects the idea that the fluxus is an inherent quality, such as a form. He disqualifies this interpretation of fluxus as ‘‘the worst possible view.’’ How then should the fluxus be understood? Oresme introduces a new ontological entity, the modus rei or way of being, to explain the phenomenon of motion. Motion is nothing but the mode or condition of the mobile object, its condition of traversing spaces in succession. The successiveness of the mobile body, however, should not be taken in the sense of a successive thing (res successiva) that is distinct from it. Thus, Oresme’s interpretation of fluxus almost turns it into a forma fluens theory.
World History
