David Lewis

(1941-2001)

The analytic language philosopher David Lewis was a possibilist. He developed the philosophical methodology known as modal realism based on the idea of possible worlds. He claims that

Possible worlds exist and are just as real as our world.
Possible worlds are the same sort of things as our world – they differ in content, not in kind.
Possible worlds cannot be reduced to something more basic – they are irreducible entities in their own right.
Actuality is indexical. When we distinguish our world from other possible worlds by claiming that it alone is actual, we mean only that it is our world.
Possible worlds are unified by the spatiotemporal interrelations of their parts; every world is spatiotemporally isolated from every other world.
Possible worlds are causally isolated from each other.

Modal realism implies the existence of infinitely many parallel universes, an idea similar to the many-worlds interpretation of quantum mechanics. In the information interpretation of quantum mechanics, quantum systems evolve in two ways: the first is the wave function deterministically exploring all the possibilities for interaction; the second is the particle randomly choosing one of those possibilities to become actual.

Possible worlds and modal reasoning made "counterfactual" arguments extremely popular in current philosophy. Possible worlds, especially the idea of "nearby worlds" that differ only slightly from the actual world, are used to examine the validity of modal notions such as necessity and contingency, possibility and impossibility, truth and falsity.

Lewis appears to have believed that the truth of his counterfactuals was a result of believing that for every non-contradictory statement there is a possible world in which that statement is true.

True propositions are those that are true in the actual world.
False propositions are those that are false in the actual world.
Necessarily true propositions are those that are true in all possible worlds.
Contingent propositions are those that are true in some possible worlds and false in others.
Possible propositions are those that are true in at least one possible world.
Impossible propositions are those that are true in no possible world .

Possible Worlds, Evil, and Free Will

In his essay, Evil for Freedom's Sake, Lewis was interested in the problem of evil as something that could be analyzed in terms of possible worlds. He examined compatibilism and incompatibilism.

Compatibilism says that our choices are free insofar as they manifest our characters (our beliefs, desires, etc.) and are not determined via causal chains that bypass our characters. If so, freedom is compatible with predetermination of our choices via our characters. The best argument for compatibilism is that we know better that we are sometimes free than that we ever escape predetermination; wherefore it may be for all we know that we are free but predetermined.
Incompatibilism says that our choices are free only if they have no determining causes outside our characters - not even causes that determine our choices via our characters. The best argument for incompatibilism rests on a plausible principle that unfreedom is closed under implication.
This sounds like van Inwagen's Consequence Argument.
Consider the prefix 'it is true that, and such-and-such agent never had any choice about whether', abbreviated 'Unfree'; suppose we have some premises (zero or more) that imply a conclusion; prefix 'Unfree' to each premise and to the conclusion; then the closure principle says that the prefixed premises imply the prefixed conclusion. Given determinism, apply closure to the implication that takes us from preconditions outside character - long ago, perhaps - and deterministic laws of nature to the predetermined choice. Conclude that the choice is unfree. Compatibilists must reject the closure principle. Let's assume that incompatibilists accept it. Else why are they incompatibilists? I'll speak of compatibilist freedom' and 'incompatibilist freedom'. But I don't ask you to presuppose that these are two varieties of freedom. According to incompatibilism, compatibilist freedom is no more freedom than counterfeit money is money.
It seems that free-will theodicy must presuppose incompatibilism. God could determine our choices via our characters, thereby preventing evil-doing while leaving our compatibilist freedom intact. Thus He could create utopia, a world where free creatures never do evil.
Plantinga once responded to compatibilist opponents as if their objection were a terminological quibble. The hypothesis is that God permits evil so that our actions may be not determined. If you find 'free' a tendentious word, use another word: 'unfettered', say. But of course the issue is one of value, not terminology. The opponents grant the value of compatibilist freedom. But they think that if God permits evil for the sake of incompatibilist freedom, what He gains is worthless.
Yet for purposes of mere 'defence' it needn't be true, or even plausible, that incompatibilist freedom has value. It is enough that it be possible. Plantinga's short way with the compatibilists would have been fair if, but only if, it was common ground that a false and implausible value judgement is nevertheless possible.
Before we turn back to the free-will theodicy that does presuppose incompatibilism, let's consider the compatibilist alternative a little further. Suppose God did determine our choices via our characters, preventing evil-doing while leaving us free. How might He do it? By a wise choice of initial conditions and uniform, powerful, simple laws of nature? - That might be mathematically impossible. The problem might be overconstrained. It might be like the problem: find a curve which is given by an equation no more than fifteen characters long, and which passes through none of the following hundred listed regions of the plane.
Rather, God might attain utopia by elaborate contrivance; Instead of uniform and powerful laws of nature, He could leave the laws gappy, leaving Him room to intervene directly in the lives of His creatures and guide them constantly back to the right path. Or (if indeed this is possible) His laws might be full of special quirks designed to apply only to very special cases. Either way, despite our compatibilist freedom, God would be managing our lives in great detail, making extensive use of His knowledge and power.

Lewis and Peter van Inwagen's Consequence Argument

In a 1981 article in Theoria, David Lewis said that van Inwagen's Consequence Argument fails as a reductio ad absurdum argument. Van Inwagen agreed and called Lewis' article “the finest essay that has ever been written in defense of compatibilism – possibly the finest essay that has ever been written about any aspect of the free will problem”. ("How to Think about the Problem of Free Will”, Journal of Ethics (2008) 12, 337-341).

Kadri Vihvelin has written a critical analysis of van Inwagen and Lewis's reply. She says "The Consequence Argument was supposed to show that if we attribute ordinary abilities to deterministic agents, we are forced to credit them with incredible past or law-changing abilities as well. But no such incredible conclusion follows. All that follows is something that we must accept anyway, as the price of our non-godlike nature: that the exercise of our abilities depends partly on circumstances outside our control."

Papers by Lewis

Are We Free to Break the Laws?, Theoria 47 (1981), 113-121) [PDF]

A Pragmatic Conception of the A Priori

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