Motion

MOTION. - Motion in general, or change, is the passage from one mode of being to another: in this sense everything moves, except God, pure act.

The Eleatic school denied all motion (v. ELEATIEMO). Zeno, supposing local motion to be composed of infinite parts, demonstrated it to be absurd with the four aporias: dichotomy, Achilles, arrow, stadium (cf. Aristotle, Phys., VI, 9). Leucippus and Democritus affirmed the local motion of atoms in the void; this is the only possible motion (id., De gen. et corr., I, 8, 325a). Aristotle admitted motion in general or change (μεταβολή, elsewhere κίνησις, Phys., III, 1, 200 b 33 ff., etc.), which is the passage from one mode of being to another: πάσα μεταβολή δότιν ἐκ τινος εἰς τι (ibid., V, 1, 225a 1). He explains it by means of a substrate that assumes different modifications (Met., VIII, 1, 1042 a 32 ff.; Phys., I, 7, 190 a 13). There are three species of μεταβολή: from subject (ὑποκείμενον) to non-subject (μή ὑποκείμενον) and vice versa; or from subject to subject, that is, corruption (φθορά), generation (γένεσις); and motion in the proper sense, κίνησις, which is continuous motion between two contraries through intermediate states (ibid., V, 225a 8 ff.). Bearing this continuity in mind, one understands the Aristotelian definition of κίνησις: act of that which is in potency as such: ἡ τοῦ δυνάμει ἔντος ἐντελέχεια, ἡ τοῦτόν (ibid., III, 1, 201 a 10). That is, motion is a perfection which postulates a further perfecting. This exigency is precisely what indicates the passing, which is essential in motion: for example, a body that moves at point P neither begins to move at P, nor ends its motion at P, but comes from a preceding point and goes to a following point; therefore it is not in P, but passes through P. Aristotle rejects the Zenonian aporias, observing that the parts of motion are not in act but in potency: if you make the parts «in act, you will not make a continuous motion but will stop it» (ibid., VIII, 8, 263a 29-30). Motion in the proper sense is possible in quality (alteration, ἀλλοίωσις), in quantity (increase, αὔξησις and decrease, φθίας), in position (local motion, φόρα; cf. ibid., V, 2, 226a 24-25; H. Bonitz, Index aristotelicus, 449a 50 - b 52).

From motion derive the categories of action and passion: «Motus, secundum quod praedicatur de subiecto in quo est, constituit praedicamentum passionis. Secundum autem quod praedicatur de eo a quo est, constituit praedicamentum actionis» (St. Thomas, In Met., XI, lect. 9, ed. Cathala, n. 2313; cf. Aristotle, Phys., III, 3, 202-213 ff.). In life and in knowledge, motion has a more perfect sense (De An., II, 5, 417 b 2-3).

Local motion is important. Aristotle defined it as essentially relative to place; therefore not even God can produce an absolute motion, that is, one not referred to a term. This conclusion was proscribed by the Archbishop of Paris, Stephen Tempier (1277); hence some later philosophers and theologians admitted an absolute local motion, which is an intrinsic changing, without relation to a term. Thus Scotus (Quodl., q. XI; ed. L. Wadding, XII, Lyon 1859, p. 263); G. Buridan (Phys., III, q. 7; ed. Paris 1509, f. 50) and their schools. William of Ockham says that a real local motion relative to a hypothetical term is possible (Quodl. VII, q. 6). F. Suárez admits absolute motion as the mutation of an absolute presence (Disp. Met., dist. 51, sect. 1, n. 21; ed. Paris 1856-61, vol. XXVI, p. 978).

According to Aristotle, local motion is a becoming, and therefore requires a mover distinct from the moved object; it is the motive cause and, when this no longer reaches the moved object, the «surrounding medium», which has received the motive force from the mover (Phys., VIII, 10, 267a 2-5). John Philoponus, a commentator of the Alexandrian school, affirms that this motive force, called «impetus», is instead produced by the mover in the moved object itself (Phys., IV, 8; ed. Vitelli, Berlin 1888, p. 642). Philoponus's theory was taken up by Francis of Marchia (IV Sent., princ.) and especially by G. Buridan, who maintained, following experimental observation, that the impetus remains unchanged and therefore the motion lasts indefinitely (Phys., VIII, q. 12; ed. Paris 1509); which is a first hint of the law of inertia. This doctrine became common in later Scholasticism (cf. R. Masi, Intorno al movimento locale, in Divus Thomas Plac., 53 [1950], pp. 3-20, 178-208). Descartes affirms that local motion is translation: «translatio unius partis materiae sive unius corporis ex vicinia eorum corporum, quae illud immediate contingunt et tanquam quiescentia spectantur, in viciniam aliorum» (Princ. Phil., II, 25, ed. C. Adam and P. Tannery, Oeuvres, VIII, Paris 1905, p. 53). For Descartes local motion is relative and reciprocal: it is an attribute of matter; all the activities of bodies are reduced to local motion (mechanism); even secondary qualities are produced in us by local motion. Moreover, local motion is not a becoming but a state, like the figure of a body, and therefore is conserved indefinitely: thus the law of inertia would be proved (Princ. Phil., II, 27, ed. cit., p. 55).

Characteristic is Newton's affirmation of an absolute local motion, «translatio corporis de loco absoluto in locum absolutum» (Philosophiae naturalis principia mathematica, Amsterdam 1723, p. 6); an absolute place which is in turn a part of absolute space (ibid.).

H. Bergson maintains that the intelligence breaks up motion in general, or change, into immobile states, as Zeno said. To re-establish the continuity of change, Aristotle admits a substrate of it (L'évolution créatrice, Paris 1912, p. 323-53). But intuition shows what change truly is, pure change: «Il y a des changements, mais il n'y a pas, sous le changement, des choses qui changent: le changement n'a pas besoin d'un support. Il y a des mouvements, mais il n'y a pas d'objet inerte, invariable, qui se meuve: le mouvement n'implique pas un mobile»: La pensée et le mouvement, Paris 1934, p. 185). Pure change is «Duration» (Durée), which is reality itself.

In physics, local motion is essentially relative to the term of reference; its laws are studied by mechanics.

Roberto Masi