Journal of the History of Ideas

Journal of the History of Ideas 61.4 (2000) 539-560

Plenitude, Possibility, and the Limits of Reason: A Medieval Arabic Debate on the Metaphysics of Nature

Taneli Kukkonen

In a recent article Simo Knuuttila has examined the argumentative patterns of modern cosmology, especially the search in fundamental physics for an "ultimate explanation," a unified "Theory of Everything" that would subsume all more local theories under its aegis. Knuuttila goes on to compare contemporary modes of explanation with previous ways of conceiving of the metaphysics of understanding. As it turns out, on the cosmological borderline where physics shades into metaphysics desires similar at heart have fueled the efforts of thinkers throughout the history of western thought. When what is at stake is the final intelligibility of the world, shared ideals of necessity, transparency, and simplicity come to the fore. More often than not, theological concerns and preconceptions accompany these notions. ¹

One can react to this finding in one of two ways, either by retorting that the moderns have not gotten very far or by pointing out that earlier discussions on the topic were often quite philosophically sophisticated. In this article I propose to investigate how some of the issues raised by Knuuttila figure in a specific historical context. This is the debate on the pre-eternity of the world initiated by the celebrated Muslim theologian Abû Hâmid al-Ghazâlî (1058-1111) in his polemic arguing for The Incoherence of the Philosophers (Tahâfut al-falâsifa, 1095) and later picked up by the Andalusian philosopher Averroës (1126-98) in [End Page 539] his reply, The Incoherence of the Incoherence (Tahâfut al-tahâfut, 1180). ² The exceptionally rich cosmological materials could be taken in any number of directions, but I will focus on three main issues: the discussants' construction of the domain of possibilities (i.e. the relation of the possible to the actual), their understanding of the reasonability of creation, and the question of its finitude and infinity. The study is undertaken partly to illustrate the sophistication with which the topics were handled in medieval Islamic philosophy, but in each case I will also draw parallels with certain modern conceptions, and I submit that in each case al-Ghazâlî's and Averroës's debate throws light on the contemporary discussion. In the final section I will develop this contention further by pointing to a connection between finitude and possibilities which I believe has both heuristic and existential value.

Plenitude and Possibility

Among historians of philosophy it is now acknowledged that in late medieval discussions certain modal theoretical innovations were instrumental in bringing about profound change in the western worldview. A conceptual shift occurred where, roughly speaking, a single intelligible universe was exchanged for several possible worlds. In Knuuttila's estimation this change in worldviews was in some ways more fundamental than anything modern science has produced since. ³ This claim deserves consideration but I would like first to outline what the change was about, for in al-Ghazâlî's and Averroës's debate this shift in modal paradigms is clearly discernible.

Representative of the philosophers' point of view, Averroës holds onto the received Arabic Aristotelian interpretation of setting possibilities in a statistical temporal-frequency framework. According to this account, what is truly possible must at some time be realized--this is a technical interpretation of what has after Lovejoy commonly become known as the principle of plenitude. ⁴ As a [End Page 540] corollary, what always is is necessarily. The cosmological consequences of these assumptions can be outlined as follows. First, lest God's creative possibilities be limited, it must be admitted that the world could have been created however long ago in the past. By extension God must have been able to create the world an infinite time ago. But if it is a genuine possibility that the world is eternally old, then according to the principle of plenitude, this possibility is an actual fact. Once it is...