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Modal realism posits the reality of all logically possible worlds. This seems to radically dispense with Occams razor by allowing the reality of all logically feasible explanations. But on reflection doesn't it also embody it by being democratic and egalitarian in its method of rehabilitating all possible explanations, that the simplest explanation is that anything goes that isn't outright forbidden by the laws of logic?

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I'm tempted to say Leibniz and the monad might be helpful in really getting our arms around this. In other words, I feel like we would need a notion of compossibility of the various aspects of physical-logical 'substrates' of a world in order to regain a selection principle. –  Joseph Weissman Feb 13 '13 at 0:27
Great question and +1, by the way. (I wonder if you might share a little bit more about why it might have become interesting to you?) –  Joseph Weissman Feb 13 '13 at 0:29
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This is something I am well acquainted with as my advisor is one of the few living, breathing modal realists.

You have to be clear on what Ockham's Razor is and is not. How to understand the principle is still a hotly debated subject.

The core idea is that, all other things being equal, the simplest theory is best. That begs for an explanation of the relevant sort of simplicity.

Generally it is qualitative and NOT quantitative parsimony that people are interested in. That is to say, the simplest theory is the one with the fewest kinds of entities and not the fewest number. Lewis says as much in his Counterfactuals:

I subscribe to the general view that qualitative parsimony is good in a philosophical or empirical hypothesis; but I recognise no presumption whatever in favour of quantitative parsimony. (Lewis 1973, p. 87)

But even that is not quite right. People tend to not be too concerned with having a lot of derivative kinds of entities. For example, should a materialist theory that defines mental processes in terms of physical processes count as equally complex to a dualist theory that takes mental processes as irreducible primitives? Many people have thought, no, the materialist has the simpler theory since he postulates the fewest number of basic or fundamental kinds of entities (in Lewis's terminology, the fewest number of perfectly natural kinds).

With that out of the way, Lewis embraces Ockham's Razor in this very Quinean form and he argues for Modal Realism precisely on the grounds of its simplicity (and fruitfulness). His claim is that he can use possible worlds to reduce modality, dispense with properties, provide analyses of causation, counterfactuals, persistence, and so on for a VERY long list of topics of philosophical interest. The reductions possible get him a simpler theory than competing theories. In fact, only a modal realist can claim to reduce modality and not take it as a theoretical primitive.

Connecting this to explanation, he is able to explain a wide array of phenomenon through his analyses. Since these analyses require only the notions of modal realism (plus the background theory he needs for modal realism), Lewis is able to provide a large number of explanations from a rather simple base. In fact, this is Lewis's claim of the fruitfulness of modal realism.

I fear I haven't fully addressed your concerns about possible/feasible explanations, but I have to admit I don't fully understand what you're asking. If you would like more information, try to clarify your concerns in a comment and I'll update my answer.


Connected to my previously stated fear, I should clarify that my answer focuses on an ontological construal of Ockham's Razor. When Lewis claims he has a simple theory, he is claiming something like the following:

Other things being equal, if a theory T_{1} is ontologically simpler than a rival theory T_{2} then it is rational to prefer T_{1} to T_{2}.

The further claim Lewis makes (see the above quote from Counterfactuals) is that it is qualitative parsimony which is the relevant sort of ontological simplicity.

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