As I wrote part of this post to explicitly discuss Mitchell Porter's position, I think it only fair to post Mitchell Porter's comment here, where it should be more at home than in "On Philosophers". -- Eliezer Yudkowsky
Mitchell Porter commented:
Before I get lost in these semantic and epistemic complexities, I will say once again what the problem is.
We are endeavoring to interpret the wavefunctions or state vectors of quantum mechanics: to form a hypothesis about the reality they describe. The hypothesis is: before decoherence there is one "quantum world", after decoherence there are many quantum worlds. As the difference between "one world" and "more than one world" is discontinuous, but the process of decoherence is continuous, with no sharp boundary between before and after, I asked exactly where the transition from one world to more than one world occurs. The reply was that that is not an issue, since the answer would make no difference to the argument in the papers. I conceded that it makes no difference to this particular argument, but the issue itself must be faced; the existence of these worlds, if they are to be taken seriously, must be an objective matter.
Somehow, having attempted to argue for that last proposition, I find myself being asked to define what I mean by "existence", to accept that someone's existence can be "vague", and who knows what else is going to come up. I accept the desirability of trying to elucidate fundamental concepts as thoroughly as possible. But can I first ask: If a person said that according to their theory of the universe, at one time you have one of something, and later on you have many copies of that same thing, but there's no particular moment in time when the one becomes the many, and that doesn't matter because the something only has a vague, fuzzy existence... wouldn't you think that the theory might have a few problems, or at least be missing a part?
Everything I have said about worlds, and observers in worlds, and about the certainty of one's own existence as an individual observer, has been meant to drive that home. That chain of relationships is the detailed reason why it is unacceptable to have a blase attitude towards the conditions of existence of quantum worlds. They must be regarded as existing or not existing, in a completely objective, absolute, non-relative way, or the concept becomes a nonsense, because worlds must play the role of hosting entities whose existence is definitely not vague or relative, namely, us.
Does no-one understand or sympathize with this line of thought?
I will get on with the philosophy in a moment. But I ask that those of you who may find yourselves in a protracted debate with me over these tangential questions, please consider anew the foregoing argument and ask yourself whether it is desirable or even possible to settle for a vague notion of "world", given the theoretical burden it has to bear.
Caledonian asks what I mean by "existence". I confess that I am unable to define it without using a synonym, which is not much of a definition. There may be quite a few similar basic indefinables, which we nonetheless manage to talk about; "negation" may be another example. It seems that all I can do is talk about it, and hope some recognition dawns. I know I already have a disagreement with Caledonian in this matter, because earlier this month he wrote here that existence is relative and depends on the possibility of interaction, something I would never say, because it confuses existence per se with something like knowability - the epistemic grounds whereby one observer may assert of one thing that it does indeed exist. We who live now, our existence was not knowable to anyone who lived a thousand years ago. Nonetheless, we do exist, here and now, and it is a fallacy to relativize our existence, and say "we exist for each other, but we don't exist for those people in the past". It is a basic confusion of knowability with reality.
Now, Unknown, what am I to do with you? Your line is that existence is a vague concept because I cannot define it without being circular, or that I cannot define it in a way which offers a clear decision procedure for existence. My line would be that we all know perfectly well what "existence" refers to - the property of being there, the property of being a part of reality, the property of not being nonexistent - but that the metaphysical depths of its nature are not so obvious. Again, one is constantly making implicit judgements about what does and does not exist. Does al Qaeda exist? Does Xenu exist? Does the special discount on milk at the corner store still exist? Existential judgements are ubiquitous in human thought. We all possess a basic facility with the concept. Does the inability to crisply define it or place it in an ontological scheme mean that one only has a vague concept of existence? I don't think so, because I think the criterion of vagueness in a concept is that its referent, the specific thing which it designates, is underdetermined (i.e. there are several different things it might be referring to), not that the nature of the referent remains incompletely specified. I think the particular referent of the human concept of existence is unambiguously known, but the nature of that referent may be obscure to the human mind. But this is a complicated matter.
And one more time: this metaphysics is an interesting and even vital topic, but it is somewhat of a tangent from the main issue, which is the meaning of "world" in many worlds.