Ibn Sina's Asymmetric Worlds — Epoche B2
Grounds, Not Worlds: Ibn Sīnā and Lewis on the Contingency of Everything What the comparison can and cannot show It is often suggested that Ibn Sīnā's modal metaphysics offers a corrective to contemporary possible-worlds theory [1] : where David Lewis makes all worlds equal [2] , Ibn Sīnā ranks them, so his "asymmetric" picture might restore an order that Lewis's parity destroys. This review argues that the suggestion misdescribes both parties. Ibn Sīnā is not proposing rival possible worlds. He is proposing a rival account of what modality is grounded in — essence and causal dependence [3] , rather than what exists elsewhere — and a comparison that treats his hierarchy as a hierarchy of worlds compares the wrong things. Once the comparison is set up properly, it yields a result, and it is not the one the framing anticipates. Both systems preserve contingency only in a relativised form, and both make the totality of what there is non-contingent. Ibn Sīnā's asymmetry does not buy back the contingency that Lewis's parity gives up; each loses it in its own way. The claim is deniable: a defender of Ibn Sīnā can hold that possibility-in-itself is an unrelativised modal fact with no Lewisian counterpart, and the penultimate section takes that objection seriously. Ibn Sīnā (c. 980–1037): the necessary and the possible in themselves Ibn Sīnā's starting point is a distinction between what a thing is and whether it is. For anything we encounter, its nature does not settle its existence: knowing perfectly what a horse is tells you nothing about whether there are any. He therefore sorts existents by their modal status considered in themselves , that is, considered apart from anything else. Something is impossible in itself if its non-existence is required by what it is — a square circle. Something is necessary in itself if its existence is required by what it is: supposing it not to exist yields an impossibility. Everything else is possible in itself : its nature is indifferent as between existing and not. The argument of the first book of the Ilāhiyyāt of the Shifāʾ , and in a tighter form in the Ishārāt , runs from here. Whatever is possible in itself does not account for its own existence, so if it exists it has been made to exist by something else. Ibn Sīnā argues that a totality of such things cannot collectively account for itself either, and concludes that there is exactly one thing that is necessary in itself. Everything else, once its cause is in place, is necessary through another : still possible considered in itself, but no longer able to fail to be, given the cause. A correction about causation This doctrine is regularly illustrated with examples of temporal generation — a sapling given water and light becomes a tree, so the tree, once the conditions obtain, must follow. The illustration misleads, and it matters here. Ibn Sīnā distinguishes causes of a thing's becoming from causes of its being , and it is the second that carries the metaphysics. A cause of being is simultaneous with its effect and sustains it for as long as it exists; the effect's dependence is not something that happened at its beginning but something true of it at every moment. The relation between the Necessary Existent and everything else is of that kind. Read the doctrine through the sapling and the tree and one gets a story about how things come about; read it correctly and it is a claim about what holds anything in existence at all. al-Ghazālī (1095): the charge of necessitarianism The objection that matters was made within the tradition. In the Tahāfut al-falāsifa [4] , al-Ghazālī attacks the philosophers' God for acting by the necessity of its nature rather than by choice [5] . If the Necessary Existent necessarily produces the first effect, and each subsequent stage follows necessarily from what precedes it, then the world in every detail follows necessarily from what could not have been otherwise. A creator on this account does not choose; it overflows. And then nothing that happens could have failed to happen. Ibn Sīnā's answer — repeated by Ibn Rushd in the Tahāfut al-tahāfut — is that possibility in itself is untouched. Each caused thing remains, in its own nature, indifferent to existing; only relative to its cause is it necessary. al-Ghazālī's reply is the one to keep hold of: an essence's indifference to existence, considered in abstraction from everything that actually obtains, does not make anything avoidable. The question was whether the world could have gone otherwise, and "the essence of a horse does not entail horses" does not answer it. Lewis (1986): parity, and where the reduction goes Lewis's On the Plurality of Worlds takes the opposite route to the same subject. There are many worlds; they are concrete, and isolated from one another; ours is distinguished only by our being in it, since "actual" works like "here". A proposition is possible if true at some world and necessary if true at all. A principle of recombination guarantees plenitude: any way of patching together duplicates of parts of worlds yields a world, so nothing that could have been is left out. The point of the exercise is reductive. Modal facts are explained by non-modal facts about what exists. Nothing modal is left as a primitive, which Lewis regards as the theory's main advantage over rivals that must simply postulate that certain ways things could be are the genuine ones. Why the usual framing is a category error Now the comparison can be stated properly. Ibn Sīnā has no plurality of worlds and does not need one; the modal status of a thing is read off its essence and its causal situation within this order of being. Lewis has no notion of a thing's essence doing modal work; the whole apparatus of necessity in itself and through another has no translation into his vocabulary, because for him necessity is quantification and nothing else. To describe Ibn Sīnā as offering "asymmetric possible worlds" is to fit him with equipment