I’ve been a little lax about updating my blogs lately since I’m spending the summer in Cambridge England and the toils of travel plus the sparse internet access here has made posting more difficult. However, I took up reading Lewis’s “On the Plurality of Worlds” during the trip and it has inspired a great number of questions and thoughts I hope to post when I have the chance. Admittedly I have yet to finish the book and only have a cursory grasp of the literature so please excuse me if I am unknowingly repeating prior work. I always appreciate comments but I would be especially grateful if anyone with more expertise in this area could correct any mistakes or fill in any gaps.

The simplest puzzle that modal realism brings to my mind, aside from its shear counter-intuitiveness, is how it is supposed to be compatible with physical realism. Or to be more precise what grounds could a modal realist have for embracing realism about physical objects like electrons tables and chairs? So while I see no logical tension between modal realism and physical realism I do think there is an epistemic tension. In particular it seems that a modal realist has no reason to find physical realism more probable now than it is prima fascia, i.e., observations about the actual world (it follows laws, appears to have persistent mind-independent objects etc..) can’t serve as justifications for physical realism.

Overview:

The essence of my puzzle is that physical realism is justified on the principle that a single coherent explanation is more probable than postulating many brute facts, i.e., it is more likely that physical objects really exist than we just happen to have experiences as if they exist. However, modal realism would have us believe that all possible worlds are equally real, including those worlds where physical objects behave in bizarre unpredictable ways. Thus it would seem the modal realist is explaining our observations by appeal to the brute fact that this world is actual. Since the specification of which world is actual is equivalent to specifying all the true facts about this world it no longer seems as if physical realism has a preferred status as a simple explanation. Both the modal realist who believes the actual world satisfies idealism and the one who believes the actual world satisfies physical realism are equally explaining our observations by stipulating them as brute facts of existence (which world we inhabit).

Update 6/22/05: Added Overview, Comment about induction (more…)