On the subject of time, Jewish medieval philosophers were divided into two broad camps: Those who subscribed basically to the Aristotelian concept of time, and those who favored a concept that goes back ultimately to Plotinus. Included among the former are Isaac Israeli, Saadiah Gaon, Abraham ibn Daud, Maimonides, and Levi b. Gershom, and among the latter are Ḥasdai Crescas and Joseph Albo. Maimonides may be taken as representative of the first group and Crescas of the second.
Maimonides, whose discussion of time appears in his Guide of the Perplexed (notably, 1:73), accepts the definition of time laid down by Aristotle as "the number of motion according to 'before' and 'after'" (Physics 4:11, 219b). Time, therefore, is neither an independent substance nor identical with motion, although it is totally dependent upon the latter and constitutes an accident of motion, which is itself an accident of body or corporeal substances. Time, consequently, possesses only a quasi-reality. Not only is it an accident of an accident, but it is composed of a past that is gone, a future that does not yet exist, and a present that serves only as a limit between the two. Accordingly, Maimonides rejects the concept of time proposed by the Mutakallimun (see *Kalām) who, basing their thought generally on the atomism of Democritus, maintained that time is composed of time-atoms or instants, which are real entities.
Despite Maimonides' basic agreement with Aristotle on the definition of time, he rejects the latter's attempt to prove the eternity of the universe from the nature of time, and argues instead that time came into existence with the creation of the universe. Prior to creation, God existed alone in timeless eternity, for inasmuch as God is absolutely incorporeal, He has no relation to motion, and consequently none to time.
Crescas' discussion of time appears in his Or Adonai as part of his massive critique of the Aristotelian philosophy. The essential distinction between Crescas and Aristotle is that Crescas divorces the existence of time from its essential dependence upon motion. Time, rather than being an accident of motion, is the continuance or duration of the stream of consciousness of a thinking mind. Thus Crescas defines time as "the measure of the continuance [duration] of motion or rest between two instants." Time, therefore, as duration, exists independently of motion. The relation of motion to time is that the former serves to determine or measure some length or part of time. Moreover, as the duration of the activity of mind, time has no extra-mental reality, not even the quasi-reality of Aristotle.
Crescas' definition has two major theological implications. First, it is not the case, as Maimonides, for example, believes, that God cannot be described as existing in time. Since duration is a quality of mind rather than of motion and body, time can be ascribed even to an absolutely incorporeal entity such as God. Second, by similar reasoning, it may be concluded that time did not come into existence with the creation of the universe, but has existed from eternity as the duration of God's infinite consciousness.
Although Albo agrees with Crescas that duration is independent of motion, he maintains that true time is only determinate or measured duration. Hence there was no real time until the creation of the celestial spheres whose corporeality provided the motion necessary to determine a length of duration.
H.A. Davidson, Proofs of Eternity, Creation and the Existence of God in Medieval Islamic and Jewish Philosophy (1987), chps. 2–3; W.Z. Harvey, Physics and Metaphysics in Hasdai Crescas (1998), ch. 1; "Albo's Discussion of Time," in: JQR, 70 (1980), 210–38; S. Feldman, "The Theory of Eternal Creation in Hasdai Crescas and Some of His Predecessors," in: Viator (1980), 289–320; T.M. Rudawsky, "The Theory of Time in Maimonides and Crescas," in: Maimonidean Studies, 1 (1990), 143–62; W.Z. Harvey, "L'univers infini de Hasday Crescas," in: Revue de métaphysique et de morale, 4 (1998), 551–57; T. Lévy, "L'infini selon Rabbi Hasdaï Crescas (1340–1412)," in: Inquisition et pérennité (1992), 161–66; S. Stern, "The Rabbinic Concept of Time from Late Antiquity to the Middle Ages," in: G. Jaritz and G. Moreno-Riaño (eds.), Time and Eternity: The Medieval Discourse (2003), 129–45; T.M. Rudawsky, "Time and Cosmology in Late Medieval Jewish Philosophy," in: ibid., 147–62; H. Maccoby, "Crescas's Concept of Time," in: ibid., 163–70; J.T. Robinson, "Hasdai Crescas and Anti-Aristotelianism," in: The Cambridge Companion to Medieval Jewish Philosophy (2003), 403–4.