However, he has been criticized for too often directing his impressive abilities towards an ill-conceived task—the revival of armchair speculative metaphysics. I think this is a fair criticism. Lewis was very adept with logic and mathematics, but he was, as far as I can tell, insufficiently familiar with the sciences, and this shows in his metaphysics.
That said, the idea for which he is most often criticized—his modal realism—is now making somewhat of a comeback in the form of Tegmark’s Level IV multiverse hypothesis. It’s still a fairly fringe and very controversial idea, of course, but its now being taken seriously in at least some non-philosophical circles. It also appears to have some currency among some of the people working on new decision theories here at LW.
That said, the idea for which he is most often criticized—his modal realism—is now making somewhat of a comeback in the form of Tegmark’s Level IV multiverse hypothesis.
Isn’t modal realism much, much more dramatic a thesis than a mere multiverse? For example, modal realism should entail that there exists a world in which there is no multiverse, even if there is one in our world.
Well, to relate the vocabularies of two theories you need the appropriate translation manual. The manual I’m considering here would equate a “possible world” in Lewis’s terminology to a “universe” in Tegmark’s terminology. According to Tegmark, all mathematically possible universes are real in some sense, and according to Lewis all metaphysically possible worlds are real in some sense (and Lewis’s conception of metaphysical possibility seems pretty close to mathematical possibility). Lewisian worlds are spatio-temporally isolated from one another, as are Tegmarkian universes. The two theories do seem almost, if not exactly, equivalent.
On the proposed translation schema, the appropriate Lewisian analogue of Tegmark’s multiverse would be the entire landscape of all possible worlds. And so, on this translation, modal realism does not entail that there exists a world in which there is no multiverse. That would be equivalent to saying that there exists a world in which modal realism is false (i.e. the landscape of all possible worlds does not exist), which modal realism cannot entail if it is coherent.
Well argued, and fair enough. I guess the question comes down to this:
and Lewis’s conception of metaphysical possibility seems pretty close to mathematical possibility
I suppose I’m under the impression that the space of metaphysical possibility is much broader than the space of mathematical possibility. Would the two spaces be identical of a reduction if mathematics to complete and consistent logic had worked out? Does Tegmark take himself to be making an empirical claim, in asserting a multiverse?
David Lewis is generally regarded as one of the most formidable philosophers of the last century in terms of sheer intellectual firepower. I’m not aware of anyone who thinks he’s outright crap. His papers are incredibly well-written—dense, but very well argued and lucid. On topics of interest to LW: he made significant contributions to causal decision theory, the interpretation of probability, the compatibilist account of free will, physicalism about the mind, and the counterfactual analysis of causation.
However, he has been criticized for too often directing his impressive abilities towards an ill-conceived task—the revival of armchair speculative metaphysics. I think this is a fair criticism. Lewis was very adept with logic and mathematics, but he was, as far as I can tell, insufficiently familiar with the sciences, and this shows in his metaphysics.
That said, the idea for which he is most often criticized—his modal realism—is now making somewhat of a comeback in the form of Tegmark’s Level IV multiverse hypothesis. It’s still a fairly fringe and very controversial idea, of course, but its now being taken seriously in at least some non-philosophical circles. It also appears to have some currency among some of the people working on new decision theories here at LW.
Isn’t modal realism much, much more dramatic a thesis than a mere multiverse? For example, modal realism should entail that there exists a world in which there is no multiverse, even if there is one in our world.
Well, to relate the vocabularies of two theories you need the appropriate translation manual. The manual I’m considering here would equate a “possible world” in Lewis’s terminology to a “universe” in Tegmark’s terminology. According to Tegmark, all mathematically possible universes are real in some sense, and according to Lewis all metaphysically possible worlds are real in some sense (and Lewis’s conception of metaphysical possibility seems pretty close to mathematical possibility). Lewisian worlds are spatio-temporally isolated from one another, as are Tegmarkian universes. The two theories do seem almost, if not exactly, equivalent.
On the proposed translation schema, the appropriate Lewisian analogue of Tegmark’s multiverse would be the entire landscape of all possible worlds. And so, on this translation, modal realism does not entail that there exists a world in which there is no multiverse. That would be equivalent to saying that there exists a world in which modal realism is false (i.e. the landscape of all possible worlds does not exist), which modal realism cannot entail if it is coherent.
Well argued, and fair enough. I guess the question comes down to this:
I suppose I’m under the impression that the space of metaphysical possibility is much broader than the space of mathematical possibility. Would the two spaces be identical of a reduction if mathematics to complete and consistent logic had worked out? Does Tegmark take himself to be making an empirical claim, in asserting a multiverse?