If anything exists it stands to reason that therefore everything that can* exist must exist, otherwise there must be a reason why something exists and something else does not.
Doesn't this invert Ockham's razor by demanding that anyone who would claim that a thing does not exist provide some proof that it is not even
possible? It is easy to prove that a four-sided triangle is not possible, but can we prove that, say, unicorns or Middle Earth not only
are not, but
could not be? There is no absurdity in the concept of a unicorn, but why on earth should we accept that unicorns exist merely because they are not, so far as we know, impossible?
(I don't agree with all of Ockham's conclusions, but I take his razor to be beyond dispute because it can be reduced to a tautology: 'It is not necessary to posit more entities than necessary.' In fact, I think that some of his errors are due to his misapplying his own razor: I think that he was wrong to reject St. Thomas Aquinas's notion of the intelligible species for example, not because he was wrong to reject an unnecessary hypothesis, but because something like the hypothesis of intelligible species actually
is necessary to explain human cognition. The principle of parsimony ('Do not posit more entities than necessary') must be compensated and complemented by the principle of adequacy ('Do not posit fewer entities than are sufficient').)
The multiverse hypothesis is also catastrophic for ethics since, as Kant pointed out, ought implies can, meaning that, by this hypothesis, everything that I ought to do has already been done, by me or my alter ego in some alternate universe, by the very fact that it is not impossible, so it makes absolutely no difference what decisions I make in
this universe. Finally, even if the sole necessary condition of a thing's existence was that it is not impossible, still the multiverse itself could not exist because it is no more possible than a four-sided triangle: if our universe does not contain everything that exists, then it is not a universe except in the trivial sense that a basket is the 'universe' of everything that exists
and is in the basket, but if the multiverse
does contain everything that exists, then it is not 'multi' in any meaningful sense.
Why do things more or less function without error? Why does the universe "work" so well, as in it works to the degree that the laws of physics are upheld? Further, we can only know something does not work right by all the other instances where things do work. In other words, the number of things that work compared to how much they don't work is quite striking. Nothing about the universe proper needs to exist without error.
How do you explain this?
I don't know if this is what you mean by the universe's "working", but, to my mind, the unity and intelligibility of the cosmos is hands down the best argument that God exists (I do not say argument 'for' God's existence, as if were a resolution to be decided upon). Anti-theists occasionally complain about theists who take refuge in the 'gaps' of human knowledge, but is it not a miracle that we can know anything at all, let alone decipher the laws of nature? Evolutionary theory can't explain this: as C.S. Peirce (one of the most rational man who ever walked the earth) clearly saw, if evolutionary materialism is true, then, at best, we can be said to 'know' what it is useful or evolutionarily advantageous for us to
believe, but if we do actually
know things in the full-blooded sense that the knower knows himself by containing within himself, at least intentionally, the object of this very knowing (and I think that we do), then, by modus tollens, evolutionary materialism cannot be true.
The inadequacy of materialism does not, however, prove that there is a God. The 'unity and intelligibility' argument instead relies on the presence of proportionality and hierarchy in the universe, and especially the proportionality between the material universe and the human mind (made possible by the substantial union, in the human person, of the spirit and the body, of which the latter is part of the material universe, and the parallel complementarity of sensibility and intellection) that makes it all intelligible, of which the most striking evidence is what the physicist Eugene Wigner dubbed "the unreasonable effectiveness of mathematics in the natural sciences" (thus, ironically, the very existence of modern, essentially Pythagorean science, which was supposed to supplant religion, is an inconvenient fact for the proponents of anti-theistic 'scientism'). Though I can do no more than sketch it out here, this, I think, is the best argument for theism, and is moreover purely 'natural-theological' in the sense that it can be used by any religion (though it is by now obvious to everyone which religion I identify with). Nor does this conflict with my refusal to sacrifice theological "details" for the sake of ecumenism, for, as St. Thomas Aquinas argued,
that God is can be known rationally, though
what he is can be known only by faith, and in this life, only in part, as if "through a glass in a dark manner."