Time

TIME. - It is, in general, the interval between two successive events and is often imagined as a homogeneous and immutable flux, in which facts happen; more precisely, time is the duration of mutable things. A distinction is made between a real time, which is the very duration of reality; physical time, studied by physics and measured with clocks; biological time, which determines the growth and decline of living beings; psychological time, lived in consciousness; absolute or imaginary time, a pure incorporeal, immutable, unlimited flowing, independent of other things. Extremely difficult to define (cf. St. Augustine, Conf., XI, 14 "What then is time? If no one asks me, I know: but if I wish to explain it to one who asks me, then I do not know"; cf. B. Pascal, De l'esprit géométrique, ed. L. Brunschvicg, Paris 1897, p. 170), time raises the same problems as space, giving rise to three opinions: subjectivists (time is subjective), ultra-realists (time is a reality existing independently of other things), moderate realists (time depends on the existence of mutable matter).

I. PHILOSOPHY

In Greek philosophy time was conceived in a realistic sense. Pythagoras thought of time as "the sphere of that which encompasses all things" (H. Diels, Die Frag. d. Vors., Berlin 1951, B, 33; cf. R. Mondolfo, L'Infinito nel pensiero dei Greci, Florence 1934, p. 46). Plato links mutability with time and immutability with eternity (Tim., 37 d ff.). According to Aristotle, time is an aspect of the continuous motion (κίνησις) of bodies, on which he grounds its subjectivity. Time is not motion, because there exist many movements, whereas time is one; moreover motion is more or less fast, but time is not (Phys., IV, 10, 218 b. 9 ff.). Nevertheless time depends on motion; and since magnitude is continuous, the motion that traverses it and the time filled by the motion are also continuous (op. cit., IV, 11, 219 a 10 ff.). There exists a before and an after in position (here and there); therefore also in motion (passing through a point near to and then far from the starting point); when we notice the before and the after in motion, that is, when we distinguish two points in motion and an interval between them, we recognize time. Which is therefore "the number" (measure) of movement according to the before and the after; ἀριθμός κινήσεως κατὰ τὸ πρότερον καὶ ὕστερον (ibid., 219 b 1). Time is then the succession as such, the passing as passing of movement. Time is not the number with which we number, but that which is numbered (ibid., 219 b 8). There is only one time for all things, because the number that measures all movements is one (op. cit., IV, 13, 223 b 3-12). Events are in time insofar as they are measurable by means of it: they are therefore in time as bodies are in place (op. cit., IV, 12, 221 a 9 ff.). Since time is the measure of motion it is also the measure of rest. (ibid., 221 b 7 ff.). Time is objective; if motion exists, time also really exists (op. cit., IV, 14, 223 a 21-29). The Stoics say that time is incorporeal, the interval of the motion of the universe (Diogenes Laertius, VII, 1, 141). Zeno affirms that "time is the interval of motion; and together it is the soul and the norm of the speed and slowness of individual beings; and in conformity with it all things begin, end, and exist" (I frammenti degli stoici antichi, edited by N. Festa, I, Bari 1932, p. 86). For Epicurus time accompanies every movement (Stobaeus, Ecl., I, 8, 252). Lucretius observes that "tempus item per se non est", but it is together with the unfolding of things (De rerum nat., I, 459 ff.). Neoplatonism understands time in a subjective sense. Plotinus links time not to the physical world, but to the soul and to eternity; the universal soul, in producing the corporeal world, places it in time, which it itself creates, while "it produces its acts one after another, in an ever-varied succession. Time is the life of the soul consisting in the movement by which the soul passes from one state of life to another state of life" (Enn., III, 7, 11). Time is then "a progressive lengthening of the life of the soul" (ibid., 12).

Time receives particular consideration in St. Augustine, who, following in the wake of Plotinus, has certain subjective elements: "If nothing passed there would be no time; and if nothing came there would be no future; if nothing were now there would be no present" (Conf., XI, 14). "Time is a certain extension" (ibid., 23). "Time is not the motion of a body" (ibid., 24). With time we measure the motion and the rest of the body (ibid., 24). But we also measure time: we measure it in our spirit, in which we find the expectation of the future, the attention of the present and the memory of the past (ibid., 27, 28). Then we measure "the impression, not the reality of things; in measuring that we measure time" (ibid., 27). Aristotle had affirmed that time is "number numbered" (Phys., IV, 11, 219 b 7-8) and then added that time is one because the number that measures all movements is one (ibid., IV, 14, 223 b 3-12), thereby coming to say that time is "number numbering". The same difficulty returns when he establishes the unity of time with the motion of the heaven, which measures the other movements (ibid., 223 b 12, ff.). To obviate these incongruities the medieval commentators deepened Aristotle's definition of time. Having defined time with Aristotle, Averroes writes that "nos non percipimus motum neque tempus nisi cum percepimus nos transmutari, quia sumus in esse transmutabili" (Phys., IV, II, comm. 98, digressio, ed. Venice 1560, p. 256). But such perception of our essential possibility of changing occurs "quando sentitur quicunque motus, sive fuerit imaginatus sive existens" (ibid., p. 254). On the other hand we cannot feel our "esse in esse transmutabili" except dependently on the motion of the heaven (ibid., p. 255); with this Averroes links time to the movement of the heaven. St. Albert the Great thinks that time is the measure of movement, primarily of the movement of the heaven, to which time refers intrinsically as to its own subject, while it is the extrinsic measure of other movements (Phys., I, IV, trac. 3, c. 17). St. Thomas takes up Averroes and St. Albert: time is "numerus motus secundum prius et posterius. Et est numerus numeratus et non numerans" (I Sent., d. 19, q. 2, a. 1): measure of movement and intrinsic measure. The unity of time depends on the movement of the heaven, to which it is intrinsic as an accident to its own subject, while it is the extrinsic measure of other movements (Sum. Theol., I, q. 10, a. 6), as St. Albert the Great said. In the Physica St. Thomas makes the unity of time depend on the motion of the heaven, perceived through our "esse transmutabile" manifesting itself in every motion (IV, lectio 17), as Averroes said, whose passage cited here the Angelic Doctor summarizes; however in this and in later works he no longer alludes to the relation of time to the motion of the heaven in St. Albert's sense, perhaps seeing in it an incoherence and a certain subjectivism. In later Scholasticism time is explained with a tendency toward subjectivism. Scotus affirms that time and motion are two ways of considering the same thing (De rerum principio, q. 18, a. 1, n. 4; ed. Wadding, Lyon 1549, p. 161). Therefore time, in the material sense, that is, insofar as it is the same as motion, is in reality; in the formal sense, it is only in the soul, subjective (loc. cit., a. 2, n. 16; p. 164; cf. a. 3, n. 32; p. 168). And since motion is really the same as time, then "quot sunt motus, tot etiam tempora realiter" (loc. cit., q. 20, a. 2, n. 16; p. 182). The unity of times is established by means of the movement of the heaven, whose intrinsic time is the single, universal, extrinsic measure of the other motions and of the other times (loc. cit., n. 19; p. 183). G. of Ockham too insists on the subjectivity of time, which properly belongs to the motion of the heaven, with which we measure the other motions (Summulae, 4, 4); and in itself it is nothing beyond motion, which in turn is nothing beyond the movable body (ibid., 3, 5). F. Suárez, following Scotus and Ockham, affirms that time is the "duratio rerum successivarum, quae successiva etiam sit" (Disp. Met., d. 50, s. 8, n. 4); it is really the same as motion (loc. cit., s. 9, n. 1), therefore every movement has its own time (loc. cit., s. 8, n. 6); time is divided into corporeal, which is the duration of the continuous and successive movement of a body, and spiritual, which is the duration of the continuous and successive local motion of a spirit (loc. cit., s. 8, n. 7). Besides the time intrinsic to every motion, there is a time that is internal to the movement of the heaven and the extrinsic measure of the other movements (loc. cit., s. 10, n. 11). In this sense time is "numerus motus".

R. Descartes states that time "which we distinguish from duration taken in general, and which we say is the number of motion, is nothing other than a certain mode by which we think of this duration"; time therefore, "outside the true duration of things, is nothing but a mode of thinking" (Princ. phil., I, n. 57); for P. Gassendi time is neither substance nor accident, yet it is nonetheless real (Physica, I, 1, 2, c. 1; ed. Lyon 1658, p. 182); it is an "extensio incorporea, fluens," in which the past and the future are contained (loc. cit., c. 7; p. 220 b); in short, it is absolute time, analogous to absolute space (ibid., p. 224 b). I. Newton writes that "tempus absolutum, verum, mathematicum, in se et natura sua absque relatione ad externum quodvis, aequabiliter fluit, alioque nomine dicitur duratio: relativum, apparens et vulgare est sensibilis et externa quaevis durationis per motum mensura" (Philos. natur. principia mathem., Amsterdam 1723, p. 5). Moreover Newton (op. cit., p. 483) and more clearly S. Clarke hold that time, like space, is a necessary consequence of God's existence and one of his attributes, without which God would not be eternal (Quatrième réplique de M. Clarke, n. 10; ed. L. Dutens, Geneva 1768, vol. II, part 1, p. 136; cf. pp. 176, 177). G. Leibniz points out the absurdity that "Dieu sera en quelque façon dépendent du temps... et en aura besoin" (Cinquième écrit de M. Leibniz, n. 50; ed. cit. p. 154; cf. p. 151); he insists that time is "purement relatif," it is "un ordre de successions" (Troisième écrit de M. Leibniz, n. 4; p. 121), the order of inconstant possibilities which therefore have a connection" (ed. cit., p. 91). Absolute time, which is outside temporal things, is nothing (Quatrième écrit de M. Leibniz, n. 16; p. 130. Cf. pp. 121, 166). G. Berkeley too criticizes Newton's absolute time and maintains that "time is nothing, when abstraction is made from the succession of ideas in our mind" (Treatise on the principles of human knowledge, n. 98). According to D. Hume, time "derives from the succession of perceptions of every kind, whether these be ideas or impressions, and impressions of reflection or of sensation" (Treatise on human nature, I, I, part 2, s. 3, n. 3; Italian trans., Bari 1926, p. 56); therefore "it is impossible to conceive... a time without succession or change" (loc. cit., s. 4, n. 1; ed. cit. p. 61). In E. Kant time is analogous to space (v.); "it is not an empirical concept derived from an experience," "it is a necessary representation that underlies all intuitions," "it is not a discursive or, as is said, universal concept, but a pure form of sensible intuition." Therefore "time is not something that subsists by itself, or adheres to things, as an objective determination, and that therefore remains even when abstraction is made from all subjective conditions of the intuition of those things... Time is nothing other than the form of inner sense, that is, of the intuition of ourselves and of our inner state." And "since all representations, whether they have external objects or not, still in themselves, as modifications of our spirit, belong to the inner state... so time is an a priori condition of every phenomenon in general; a condition, indeed, immediately of inner phenomena (of our soul), and therefore mediately also of external ones" (Critica della rag. pura, Italian trans., Bari 1945, p. 73 ff.). According to Hegel, the idea becomes other than itself by extending itself in space and time. "Time is the being that, while it is, is not, while it is not, is: becoming intuited. The dimensions of time — present, future, past — are becoming as such in its exteriority" (Encicl. delle sc. filos. in compendio, Italian trans., ibid. 1923, p. 214). G. Gentile states that time is "a system of the manifold" (Teoria gener. dello spirito come atto puro, Florence 1938, p. 112); "time is the spatialization of the unity of space" (op. cit., p. 115); that is, in any punto dello spazio, gli elementi dell'esperienza « si escludono a vicenda in quanto l'uno sottentra all'altro nello stesso luogo », (loc. cit.) questa successione moltiplica in senso temporale l'unità, cioè il punto dello spazio.

In contemporary philosophy, time sometimes figures as a category of the real. In his idealism with a Leibnizian background, J. Royce affirms that time is the point of passage of reality in its tendency from (from)... towards (towards)... It is a constitutive moment of the real which, insofar as it contains that tendency within itself, implies in its definition the past (from) and the future (towards). In the concretely lived present, the past and the future are present, not as represented, but in their full reality. Therefore the present is an eternity within brief limits; and if we remove the limits, we have an unlimited eternity (The world and the individual, New York 1901). H. Bergson distinguishes a temps-longueur, which is the time of science, a succession measured in space by the cinematographic method (temps spatialisé), and a temps-invention, which is the very "durée réelle," in which there is no before and after, reality itself as vital impetus grasped in our inmost being with indivisible unity (L'évolution créatrice, Paris 1912, p. 363 ff.; La pensée et le mouvant, ibid. 1934, p. 188 ff.; etc.). According to M. Heidegger, the ultimate meaning of Dasein, which is the being of man, is a process of temporalization (Sein und Zeit, Halle [Saale] 1927, p. 19, 327), and time is not a thing that is, but a development that takes place in the unity of its movements; it is that which is not, but exteriorizes itself (op. cit., p. 329). The temporalization of Dasein lies in the unity of its three ec-static moments (past, present, future, which are not three parts of time), of which the principal one is the future, insofar as time springs from the projection of a past, which arouses the present, towards the future.

Conclusione. — The critique of the notion of time is analogous to that of space (v.). Absolute time, independent of changing realities, is imaginary and absurd. Illusory is the demand that a being cannot exist except within a time presupposed by it: "nor is an absolute temporal flux necessary to guarantee the validity of physical laws, as I. Newton claimed." Divine properties would belong to an absolute time: and this is absurd. Thus Leibniz, Berkeley (op. cit., n. 98) and others saw clearly; St. Thomas observes that "ante principium mundi non fuit tempus reale sed imaginarium" (Q. d. de Pot., q. 3, a. 1, ad 10). The critique of Leibniz's phenomenal time is similar to that of space; likewise for Kant, who identifies real time with imaginary time and introduces an a priori form of time, in order to avoid the absurdities of absolute time (Crit. d. rag. pura, ed. cit., pp. 80, 86). His assertion that a science of motion is not possible without the a priori form of time is gratuitous (op. cit., p. 75). The critique of time as a category is connected with the consideration of the entire system in which it is embedded. Therefore time must be thought according to Aristotelian thought, perfected by Arab and Latin Aristotelianism. It is passing in general insofar as it is passing, the physico-chemical unfolding of the universe insofar as it is unfolding, it is the real duration of the corporeal world; not of this or that body but of a body in general, which in fact will then be this or that one. The duration of a body is not in time but, together with the duration of other bodies, constitutes real time. Time, insofar as it is real, is an aspect of the real duration of the corporeal universe, from which it is not distinct. In this sense time cannot slow down, nor accelerate, nor turn back; these properties belong to movement, that is, to passing, not to passing insofar as it is passing, which is time.

II. PHYSICS

Physical time is what physics measures. Time is measured by means of motion; the diurnal motion of the earth, which determines the day, is most suitable for precision and universality. A distinction is made between the sidereal day, which is the interval between two successive passages of a star across the local meridian; the solar day, longer than the former by about 4 minutes, which is the same interval at the passage of the sun; and the mean solar day, which is an average of the values of the solar day, variable during the year because of the elliptical shape of the earth's orbit around the sun.

In classical physics, time was conceived as uniform and unique, in which it was possible to compare all the events of the universe, however distant from one another (absolute simultaneity). A. Einstein pointed out that this absolute simultaneity must be defined by experiments (operational definition), which are possible in a static world with signals of any kind: but in the actual world, in which all bodies are in continuous relative motion, with signals of infinite velocity; which is physically absurd. The best signal is in fact the electromagnetic one, which however, by the second postulate of restricted relativity (v.), is postulated to have constant velocity (5 × 10¹⁰ cm/sec) with respect to any inertial system. From this postulate it follows that two events simultaneous in one system are not so for another in relative motion: that is, in the system in relative motion time dilates; every system has its relative time, and every point of the system has its local time. H. Minkowski (1908) linked time to space in a four-dimensional universe. In it time is homogeneous with the spatial dimensions and contains the past and the future existing together, as a whole already given, while we perceive its successive three-dimensional sections (which we call the present), which are different for observers in relative motion. Becoming then becomes illusory, because it presupposes the flowing of time, which instead is already wholly given: «There is no longer becoming in three-dimensional space, but being in four-dimensional space» (A. Einstein, Sulla teoria generale e speciale della relatività, Bologna 1921, p. 110). In the general theory of relativity time participates in the modifications of the other three dimensions of Riemannian curved space, determined by the presence of matter and energy, and depends not only on relative motion but also on the point of the gravitational field in which the clock is located.

Since motion is the unfolding of an act, at every instant the transient actualities of all bodies that move (endure) surface simultaneously into reality, and have a concrete and metaphysical comparability, which we can very well conceive. Ontological comparability grounds absolute ontological simultaneity. Therefore the relativity of simultaneity is only experimental, inasmuch as it is not possible to compare distant facts that occurred in systems in relative motion, except by signals, which have a finite velocity, specified by the postulate of the constant velocity of light with respect to every system in inertial motion. On this postulate the relativity of simultaneity essentially depends. Hence the time of relativity can have only a symbolic and systematic value: the reciprocity of the modifications of space-time for two observers in relative motion cannot have ontological meaning; all the more so since, as was said for space (v.), the postulate of the constancy of the velocity of light has not been positively demonstrated and the principle of equivalence is still a hypothesis. Insofar as the time of relativity is confirmed by experience, it unites in mathematical formulas the readings of measuring instruments, without claiming to explain the ontological reality of time.