The best talk I heard at the International Congress of Logic Methodology and Philosophy of Science in Beijing was, somewhat to my surprise, the Presidential Address by Adolf Grünbaum. I wasn't expecting much, as the genre of Presidential Addresses by Octogenarian Philosophers is not one noted for its moments of soaring rhetoric. I recognized Grünbaum's name as a philosopher of science, but didn't really know anything about his work. Had I known that he has recently been specializing in critiques of theism from a scientific viewpoint (with titles like "The Poverty of Theistic Cosmology"), I might have been more optimistic. Grünbaum addressed a famous and simple question: "Why is there something rather than nothing?" He called it the Primordial Existential Question, or PEQ for short. (Philosophers are up there with NASA officials when it comes to a weakness for acronyms.) Stated in that form, the question can be traced at least back to Leibniz in his 1697 essay "On the Ultimate Origin of Things," although it's been recently championed by Oxford philosopher Richard Swinburne. The correct answer to this question is stated right off the bat in the Stanford Encyclopedia of Philosophy: "Well, why not?" But we have to dress it up to make it a bit more philosophical. First, we would only even consider this an interesting question if there were some reasonable argument in favor of nothingness over existence. As Grünbaum traces it out, Leibniz's original claim was that nothingness was "spontaneous," whereas an existing universe required a bit of work to achieve. Swinburne has sharpened this a bit, claiming that nothingness is uniquely "natural," because it is necessarily simpler than any particular universe. Both of them use this sort of logic to undergird an argument for the existence of God: if nothingness is somehow more natural or likely than existence, and yet here we are, it must be because God willed it to be so. I can't do justice to Grünbaum's takedown of this position, which was quite careful and well-informed. But the basic idea is straightforward enough. When we talk about things being "natural" or "spontaneous," we do so on the basis of our experience in this world. This experience equips us with a certain notion of natural -- theories are naturally if they are simple and not finely-tuned, configurations are natural if they aren't inexplicably low-entropy. But our experience with the world in which we actually live tells us nothing whatsoever about whether certain possible universes are "natural" or not. In particular, nothing in science, logic, or philosophy provides any evidence for the claim that simple universes are "preferred" (whatever that could possibly mean). We only have experience with one universe; there is no ensemble from which it is chosen, on which we could define a measure to quantify degrees of probability. Who is to say whether a universe described by the non-perturbative completion of superstring theory is likelier or less likely than, for example, a universe described by a Rule 110 cellular automaton? It's easy to get tricked into thinking that simplicity is somehow preferable. After all, Occam's Razor exhorts us to stick to simple explanations. But that's a way to compare different explanations that equivalently account for the same sets of facts; comparing different sets of possible underlying rules for the universe is a different kettle of fish entirely. And, to be honest, it's true that most working physicists have a hope (or a prejudice) that the principles underlying our universe are in fact pretty simple. But that's simply an expression of our selfish desire, not a philosophical precondition on the space of possible universes. When it comes to the actual universe, ultimately we'll just have to take what we get. Finally, we physicists sometimes muddy the waters by talking about "multiple universes" or "the multiverse." These days, the vast majority of such mentions refer not to actual other universes, but to different parts of our universe, causally inaccessible from ours and perhaps governed by different low-energy laws of physics (but the same deep-down ones). In that case there may actually be an ensemble of local regions, and perhaps even some sensibly-defined measure on them. But they're all part of one big happy universe. Comparing the single multiverse in which we live to a universe with completely different deep-down laws of physics, or with different values for such basic attributes as "existence," is something on which string theory and cosmology are utterly silent. Ultimately, the problem is that the question -- "Why is there something rather than nothing?" -- doesn't make any sense. What kind of answer could possibly count as satisfying? What could a claim like "The most natural universe is one that doesn't exist" possibly mean? As often happens, we are led astray by imagining that we can apply the kinds of language we use in talking about contingent pieces of the world around us to the universe as a whole. It makes sense to ask why this blog exists, rather than some other blog; but there is no external vantage point from which we can compare the relatively likelihood of different modes of existence for the universe. So the universe exists, and we know of no good reason to be surprised by that fact. I will hereby admit that, when I was a kid (maybe about ten or twelve years old? don't remember precisely) I actually used to worry about the Primordial Existential Question. That was when I had first started reading about physics and cosmology, and knew enough about the Big Bang to contemplate how amazing it was that we knew anything about the early universe. But then I would eventually hit upon the question of "What if they universe didn't exist at all?", and I would get legitimately frightened. (Some kids are scared by clowns, some by existential questions.) So in one sense, my entire career as a physical cosmologist has just been one giant defense mechanism.