g comments on A Priori - Less Wrong

32 Post author: Eliezer_Yudkowsky 08 October 2007 09:02PM

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Comment author: g 12 October 2007 12:31:00AM 6 points [-]

Richard, I would like to know what you mean by "conceptually possible" and why you think conceptual possibility has anything to do with actual possibility. I *think* you mean something like "I can/can't imagine X without any obvious inconsistencies". So, e.g., you can imagine, or think you can imagine, a world physically identical to ours in which people have no experiences; but you can't imagine, or think you can't imagine, a world physically identical to ours in which jumbo jets don't fly.

But whether something is "conceptually possible" in this sort of sense obviously has as much to do with the limits of our understanding as with what's *actually* possible, no?

1. Consider some notorious open problem in pure mathematics; the Riemann hypothesis, say. I can, in some sense, "imagine" a world in which RH is true and a world in which RH is false; I can tell you about some of the consequences in each case; but, despite that, one of those worlds is *logically impossible*; we just don't know which. (I'm ignoring, because I'm too lazy to think it through now, the possibility that RH might be undecidable.) So something can be "conceptually possible" despite being logically impossible and hence (if you believe in possible worlds) false in all possible worlds.

2. I cannot, so far as I can tell, imagine what it would be like if the world had two "timelike" dimensions and two "spacelike" ones rather than 1 and 3. (Perhaps if I sat down and concentrated for a while I could; in which case, make it twenty of each, or something.) I can calculate some of the consequences, I suppose, but I can form no coherent mental picture. None the less, it seems clear that such a world is possible in principle. So something can be (for a given person, at least) "conceptually impossible" despite being possible in other senses.

Examples like these make it seem obvious to me that "conceptual possibility" tells us much more about the limits of our imagination and reasoning than it does about the nature of reality.

You can't imagine a world physically like ours in which jumbo jets don't fly; that would be because flying is simple enough that we have some a pretty good understanding of how it works, and what mechanisms underlie it. Of course we don't have any similarly good understanding of how minds work. It seems to me that that's the *only* difference here. Lack of understanding is not evidence of magic.

(Suppose I claim that I *can* so imagine a world physically identical to ours in which boeing-arranged atoms at 10k feet aren't flying airplanes; they're, er, zairplanes; they are doing something physically indistinguishable from flying, but of course it isn't *really* flying. Those who fail to see the difference just lack sufficient subtlety of thought. Ridiculous, no?)

Anyway, let's suppose it's "conceptually possible" that the world should be exactly as it is, physically, but with no consciousness anywhere to be found. So what? All that means is that someone can form some sort of mental picture of what such a world might be like. I don't see how to eliminate the possibility that filling in the details might ultimately lead to a contradiction (as with either RH or not-RH). Or that digging further into the notion of "phenomenal consciousness" being used might reveal that it has no real content and serves only to obfuscate. (I strongly suspect that this is in fact the case. Of course that doesn't mean that those who appeal such notions have any *intention* to obfuscate.)

For what it's worth, I'm pretty sure that a zombie world is *not* conceptually possible to me: I can only "imagine" such a world by deliberately not thinking too hard about the details.