Some physical theories predict that almost all brains in the universe are Boltzmann brains, i.e. short-lived disembodied brains that are accidentally assembled as a result of thermodynamic or quantum fluctuations. Physicists and philosophers of physics widely regard this proliferation as unacceptable, and so take its prediction as a basis for rejecting these theories. But the putatively unacceptable consequences of this prediction follow only given certain philosophical assumptions. This paper develops a strategy for shielding physical theorizing from the threat of Boltzmann (...) brains. The strategy appeals to a form of phenomenal externalism about the physical basis of consciousness. Given that form of phenomenal externalism, the proliferation of Boltzmann brains turns out to be benign. While the strategy faces a psychophysical fine-tuning problem, it both alleviates cosmological fine-tuning concerns that attend physics-based solutions to Boltzmann brain problems and pays explanatory dividends in connection with time’s arrow. (shrink)

Imagine that the person you see through the looking glass is a real person, with her own experiences, living in an environment that is the mirror-reverse of yours. You look at your right-hand glove as you put it on your right hand; she looks at her left-hand glove as she puts it on her left hand. You feel your heart beating on your left side; she feels her heart beating on her right side. You hear a bird chirping out the (...) window to your left; she hears a bird chirping out the window to her right. Is there any difference between your experience—your what-it-is-like phenomenal character—and hers? She is looking at her left-hand glove, the mirror-reverse of you as you look at your right-hand glove. Does she see what you see, a right-hand glove, or the reverse of what you see, a left-hand glove? Since her environment is the mirror-reverse of yours, and she seems to get by in it just fine, you'd probably assume, if you had to guess, that she sees it correctly. She is looking at her left-hand glove, after all. So it stands to reason that she is having an experience as of a left-hand glove, not a right-hand glove. And it stands to reason—it seems obvious—that an experience as of a left-hand glove differs in what it is like from an experience as of a right-hand glove. In this paper, I will argue that this last presupposition, obvious though it may seem, is false. I'll argue that (unless dualism is true) there is no intrinsic difference between what it is like to experience a left-hand glove and what it is like to experience a right-hand glove: if there are any relevant experiential differences, these are either merely comparative (differences in what it is like to compare gloves) or cognitive (differences in what it is like to bring an experience of a glove under a concept like LEFT or RIGHT). So your mirror-reverse twin, who won't differ from you in what she is comparing or thinking, will have the same (type of) experience as you. I'll call the position I am arguing against orientation–experience categoricalism, and the position I am arguing for orientation–experience relationism. Orientation–experience relationism is a surprising thesis, one whose falsity you might take for granted. I will argue for it by cases. Things go wrong for the orientation–experience categoricalist in one sort of way if we imagine that you and your mirror-twin's experiences differ: basically, this leads to absurdity if we imagine that you and she both live at a world with a non-orientable spatial topology (in the sense in which a Möbius strip or Klein bottle are non-orientable). Things go wrong for the orientation–experience categoricalist in a different sort of way if we assume that you and your twin would experience the same, but that someone else might have the experience that we were just imagining that your twin might have. Under these assumptions, we have to accept the existence of partial zombies, who lack the relevant phenomenology but behave as if they have it, at very close, empirically possible worlds. In an appendix, I argue that property dualism is the best alternative for those who cannot bring themselves to accept orientation-experience relationism. (shrink)