I don't think there's a deep puzzle here, as I hope I can explain. The kind of possibility that stems from uncertainty is usually called "epistemic possibility," often signaled in English by "may" or "might," as in "There may [or might] be life on other planets." We're far from certain that there isn't life on other planets, so there may [for all we know] be life on other planets: life on other planets is an epistemic possibility for us. There are other kinds of possibility, such as metaphysical possibility, but I think the general point I'll make applies to all of them. To deny possibility in this sense, to say that some state of affairs S isn't epistemically possible for someone, is roughly to say that he/she is certain that S doesn't obtain, or at least he/she knows that S doesn't obtain (if knowledge doesn't require certainty). So I'd say that, right now, my own non-existence is epistemically impossible for me, because I know (indeed, I'm certain) that I exist, for the reason Descartes gives...

First a terminological quibble. By "scientifically impossible," I take it you really mean just "technologically infeasible," i.e., impossible given the limits of current technology. As I see it, what's scientifically possible or impossible depends only on the laws of nature, which are standardly regarded as unchanging over time (or at least over any time that humans will experience). I think the jury's still out on whether backward time-travel is scientifically impossible in this latter sense. To your question: I think there's something self-contradictory in the idea of "correcting what you did" if that means "bringing it about that you never did what you in fact did." Either (1) you did it, or (~ 1) you never did it. I can't see how any consistent story features both (1) and (~ 1). In that sense, then, there's no such thing as (2) "going back in time and changing things in reality" and therefore nothing that's "the same as" (2). See section 1.2 of the SEP article on time-travel .

I've seen only one of the Matrix films, the first one. You might ask your colleague how he can be certain that things in our world aren't as they're portrayed in that film: that is, you and he merely believe you're conversing in the ordinary way about an ordinary teacup, when in fact you're both hooked up to a computer that's simulating the conversation, the teacup, and your surroundings. Is there some internal indication, something about the way things feel to him during such a conversation, that rules out a Matrix-style simulation? What could that be? Granted, neither you nor your colleague are at all inclined to believe that you're living a simulated existence, but that's just how the Matrix wants it!

I think it does, or at least I think the burden of proof is on anyone who says that "exist" is systematically ambiguous, meaning one thing when applied to numbers and another thing when applied elsewhere. It's widely held that abstract objects such as numbers, if indeed they exist, don't exist in spacetime, whereas concrete objects such dogs clearly do exist in spacetime. But that doesn't affect the meaning of "exist" itself. In particular, it doesn't imply that "exist" means "exist in spacetime." Otherwise, the expression "exist in spacetime" would be redundant and the expression "exist but not in spacetime" would be self-contradictory, neither of which is the case. Analogy: It's a fact that some things exist aerobically and some things exist anaerobically, but that fact doesn't tempt anyone to say that one or the other kind of thing doesn't really exist, or to say that "exist" just means "exist aerobically." So I see no reason not to say that numbers, if they exist, exist nonspatiotemporally,...

I can't do better than to recommend that you consult the SEP entry on monism written by Jonathan Schaffer. It's not clear to me from your question whether you favor what Schaffer calls "existence monism" or instead what he calls "priority monism," but you'll find both views expertly discussed.

I think you've put your finger on a dubious feature of the KCA. While I would say that a chair does begin to exist when it's created from pre-existing materials, I agree with you that if the universe began to exist, the universe didn't begin to exist in the same sense in which a chair does. So I think you're right to detect a questionable move from "Everything within the universe that begins to exist has a cause" to "Everything, including the universe itself, that begins to exist has a cause." It's not at all clear that the phrase "begins to exist" is being used in the same way both times. To the question "You've never seen a chair pop into existence, have you?" one can reply as follows: "I've never seen anything arising from pre-existing materials pop into existence, but that isn't relevant to whether something not arising from pre-existing materials can pop into existence."

How about this: All physical things occupy spacetime. But not all nonphysical things occupy spacetime, and maybe none do. The clearest example of allegedly nonphysical things would, I think, be abstract objects such as numbers and sets. Platonists say that there are infinitely many such things. See this SEP entry .

...how can non-being, as a concept, not have being? I see no reason why it can't. I see no reason to think that the concept of non-being (or non-existence, or related concepts) fails to exist. Not every concept must instantiate itself. Indeed, typically concepts don't instantiate themselves: the concept of tree isn't a tree, etc. It's more controversial whether being (or existence) is a characteristic (or property) of things. You'll find detailed discussion of this issue in the SEP entry linked here . I myself favor the widely held view that all (and only) things exist. It's not clear to me that this view implies that existence is a property, nor is it clear to me that anything of importance turns on whether existence is a property.

Speculate is pretty much all that philosophers, as such, can do about this question. For what it's worth, however, some well-placed astronomers are confident that life (intelligent or not) does exist elsewhere and that we'll discover evidence of it within 20 years: see this link .

You might also look into the work of philosopher Eli Hirsch (Brandeis University), who argues that various disagreements in ontology, perhaps including the one you mentioned, are "merely verbal" disagreements.