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Home , On the Heavens, Sublunary sphere

CHAPTER TWO

ARISTOTLE'S CRITICISM OF 'S MATHEMATICAL

Given the dialectical character of 's philosophical method, it is reasonable to assume that his cosmological views were developed in response to those of Plato, the Pythagoreans, and other Presocratic thinkers. For instance, against the mathematical cosmology of Plato's Timaeus, Aristotle insists that it is rather than mathematics which studies the and motions of sublunary bodies. Yet he adopts Eudoxean astronomy as a guide to the motions of the heaven­ ly bodies, while also accepting optics and as true for the . For instance, Aristotle's claim that all heavy bodies fall towards the center of the universe seems to involve mathe­ matical symmetry as part of his theory of natural places. So the similarities and differences between Plato's and Aristotle's cosmology can be clarified in terms of the distinction between physics and mathematics.

I. Aristotle's response to the Timaeus dialogue

Any assessment of Plato's influence on Aristotle's cosmology must begin with the Timaeus, which is mentioned more frequently than any other dialogue.' Thus the De Caelo is the locus classicus for Aristotle's criticism of the Timaeus, since both works may be seen as parallel cosmological treatises. In this regard one must take account of the problem raised by Jaeger2 as to whether a particular treatise

1 Cf. Bonitz (1870), 598a60-b19 & 761bSS-60. 2 Jaeger (1923) thinks that, since Book I of De Caelo brings aither within the realm of physis, it must have been written after De Philosophia which he dates at 347 B.C. But this conjecture depends on the dubious claim that the latter work still subscribes to the psychology of Plato's Laws, according to which physis is subordinated to the World-Soul. Furthermore, Jaeger (1923) 299 ff. claims that the confident tone of De Caelo I, 2-3 makes it likely that a version of the five elements theory had already been proposed prior to De Philosophia and that it is now being rewritten along mechanistic lines. Such speculation is challenged by Bos (1973) 131 as inconsistent and lacking in evidence. 72 CHAPTER TWO presents an earlier or later phase in Aristotle's thought, even though it may be an impossible question to settle, if Aristotle reworked some treatises during his lifetime. 3

I. 1. The program qf De Caelo Given the discussion of the sublunary world in Books III & IV, the traditional title (i.e. '') must refer to the , since the whole work outlines a world-system which (even without an ) can be attributed to Aristotle.4 For instance, the cosmos is held to have a finite spherical body, with the fixed at its extreme circumference and the earth at its center, while the places in between are occupied by other elemental bodies. 5 Contrary to Anaxagoras, Aristotle thinks that the sphere of the fixed stars (as well as those of the planets) is not composed of but of a fifth body popularly called aither. At De Caelo 1.2 he argues for the existence of this simple body on the grounds that circular motion does not be­ long naturally to any of the sublunary elements, as their natures dictate that they move in rectilinear fashion away from or towards the cen­ ter of the universe. Aristotle assumes that the heavenly bodies move eternally in perfect circles, so that this kind of motion must belong naturally to their matter. Since none of the four elements fits the bill, there must be a fifth body which is simpler and has a higher nature because circular motion is prior to rectilinear motion. 6 Thus

3 In his 'Introduction' to the Loeb edition, Guthrie (1939) xvi describes the De Caelo as a transitional work that contains both earlier and later views. Bos (1973) 2 also holds that within this work one must distinguish earlier and later strata. 4 Solmsen (1960) 272-3n24 thinks that the first mover is introduced at 300b21 but that the concept is different from that in the Physics and , since it is said to be itself in motion by nature. However this is conceptually more akin to Plato's concept of soul as self-moving by nature than to the notion of an unmoved mover. In support of his hypothesis about the later date of De Caelo I & II, Manuwald (1989) 115 argues that the unmoved mover is clearly assumed in II.l2, and taken to be among the transcendent entities mentioned in I.9. 5 Cf. Gael. 269a30 ff., 269b 15-16. 6 For Plato circular motion is characteristic of a completely rational soul, whereas Aristotle makes it a typical sign of the nature of aither. So Bos (1973) 58 suggests that this first element is introduced by Aristotle to replace the Platonic World-Soul as the agent responsible for the uniform circular motion of the World-Body. As evidence for a transition to this view, Bos p,1ints to De Philosophia (Fr. 27) where Aristotle seems to regard aither as the substance of the soul. If this is a response to the Platonic problem of how immaterial soul can act on the body, then perhaps Bos is right to emphasize that aither is treated by Aristotle as a first body.