I don't know why you retracted this. . .

I retracted it because when I wrote it I hadn't known Tegmarkism was part of Yudkowskian eclecticism. In that light, it deserves a less flippant response. While it strikes me as being as absurd as the ontological argument, for some of the same reasons, I can dispositively refute the ontological argument; so if they're really the same, I ought to be able to offer a simple, dispositive refutation of Tegmark. I think that's possible to, but it's instructive that the refutation isn't one that applies to the ontological argument. So, contrary to what I said, they're not really the same argument. Arguably, even, I committed what Yvain (mistakenly) considers a widespread fallacy, his "worst error," since I submerged Tegmark in the general disreputability of inference from possibility to necessity.

Briefly, Tegmark's analysis is obfuscatory because:

A. The best (most naturalistic) analyses of knowledge hold that it results from our reliable causal interactions with its objects. Thus, if Tegmark universes exist, we could have no knowledge of them (which leaves us with no reason to think they do exist).

I don't know how Tegmark addresses this objection. Or even if he does, but this objection seems to me the basic reason Tegmark's constructs seem so dismissible.

B. It's easy to "solve" many metaphysical and cosmological problems by positing an infinite number of entities, whether parallel universes or an infinite cosmos, but the concept of an actually realized infinity is incoherent.

[Side question: Does anyone happen to know whether the many-worlds interpretation of q.m. posits infinitely many worlds--or only a very, very large number?]