The Universe as a System: Ibn Sīnā’s Cosmology Revisited

Abstract

Ibn Sina (d. 428/1037) is one of the most celebrated 'scientist philosophers' the Muslim world has produced. (1) Besides the influential Liber Canonis on medicine, Ibn Sina wrote Kittab al-Shifa, multi volume encyclopedic masterpiece embodying a vast field of knowledge from logic and metaphysics to mathematics, astronomy and music, which was in part translated into Latin and exerted tremendous influence in subsequent centuries. This article aims to discuss some aspects of Ibn Sina's cosmology. An outline of his picture of the physical universe is given together with an exposition of its philosophical underpinnings, followed by an analysis of his views about the nature and motion of heavenly bodies. 1. General Picture of the Cosmos Drawing on Aristotle's cosmology and Ptolemaic astronomy, Ibn Sina views the universe as consisting of nine concentric spheres contiguously nested, one within the other, from the lowest sphere of the moon to the outermost starless sphere. These spheres are thought to be concentric because they seem to share a common center, which is the center of the universe, taken as coincident with the earth's center. On this model, each of the seven known 'wandering stars' or planets (al--kawakib al-mutahayyirah)--namely, the moon, the two inner planets (Mercury and Venus), the sun, and the three outer planets (Mars, Jupiter, and Saturn)-and the 'fixed stars' (al-thawabit) are assumed to be attached to eight solid but transparent spheres that carry them as they revolve around the earth. (2) There is a ninth, outermost sphere (kurah kharijah anha muhitah), which defines the edge or boundary of the universe and supposedly contains no star (ghayr mukawkabah), posited to explain the daily motion of the heaven, whereas the motion of the eighth sphere (that of the fixed stars) is said to be due to the precession of the equinoctial points (nugtatd al-itidal). (3) Each of these spheres, according to Ibn Sina, is governed by an intelligence and a soul, which are respectively the remote cause and proximate principle of their motion. Ibn Sina's model rests on four fundamental assumptions, namely: (1) that the universe is one in number; (2) that it is finite in extent and spherical in shape; (3) that it has a center; and (4) that the earth lies at its center. Let us first consider the third and fourth assumptions. Ibn Sina argues for the central position of the earth by means of a logical argument which essentially de rives from the Aristotelian physical theory of four elements (earth, water, air, fire) and their natural motion and place. For him as for Aristotle, any motion of natural bodies (that is, anything capable of motion and change, whether animate or inanimate) is either simple or composite, natural or unnatural. Simple motion, which belongs to simple bodies (as opposed to composite bodies), is either rectilinear (mustagimah) or circular (mustadirah). Simple rectilinear motion is either motion away from the center, motion toward the center, or motion about the center. (4) Motion away from the center toward the cosmic circumference, termed upward motion, is natural to light bodies, whereas motion toward the center, called downward motion, is natural to heavy bodies. (5) The motion of a body is said to be natural (tabiiyyah) if it drives the moving body toward the place where it will rest 'naturally', that is, by nature and not by an external force, whereas unnatural motion is that which is due to some external force contrary to the thing's nature (6)--nature being identified as an intrinsic principle of being moved and being at rest. (7) Since the sub-lunar elements (anasir) are natural simple bodies (basait), their motions must be both simple and natural, (8) but also rectilinear and not circular because, in the absence of any hindrance, each of the elements will by nature either move straight up or straight down, seeking its natural place. (9) By 'natural place' (hayyiz tabii) is meant the place to which a natural body is moved or inclined to move and where it will rest naturally, (10) namely, the cosmic center for heavy bodies, and the circumference for light ones. …