I've been wrestling with this problem for some time. My question concerns the concept of 'possibility'. When one says that something is possible, they are saying that something might be but may not be as well. There is an uncertainty. And of course whatever it is cannot both be and not be at the same time. Now, when we say that something is 'not possible', we are saying that something is not and cannot be. There is no uncertainty and the term as used does not seem to be a true negation as is usually meant when the term 'not' is used. What confuses me, is that in when actually trying to negate the concept of possibility, such as when saying 'not possible', aren't we on the one hand saying that 'that which might be' is not, and on the other hand that 'that which may not be' is not as well, and therefore is (or could be)? What may be is not and/or what may not be is. Saying that something is not possible, in this sense, is the same as saying that it is possible, thus making the negation of the concept...

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...

We know for now, at least, it's impossible to go back in time scientifically. But what if you really needed to, say if you had done something really bad and had ever desperation to go back in time and correct what you did, so you don't suffer the consequences you are suffering in the present. Provided you would not cause a disaster by going back in time, and that you would only change the bad things you did, it is an interesting concept. With this context, if you could be given a drug, that would leave you asleep for the rest of your life (coma), would you do it? Read on, there's more. In this sleep, you will have a dream, which is set from just before your mistake. So essentially, it causes you to simulate the past and the rest of your life in your head. It seems real, but it isn't. My question is, would this be the same as going back in time and changing things in reality? Does reality matter more, or our interpretation of it?

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 had a brief chat with a work colleague today about the nature of reality and our perception of it. Essentially, his contention was that because we all basically agree on our external physical reality (e.g. when I hand him a cup of tea we both agree that I've just passed him a cup of hot tea), there must be an external reality because we both seem to agree on what it's like. If there wasn't such an external reality and we didn't essentially agree on it, he pointed out, we wouldn't be able to even ask for a cup of tea because my idea of what a cup of tea actually is would be totally different (or at least different enough to make meaningful communication difficult). Therefore, he concluded, it's common sense that we must be talking about and looking at the same "real" things and that we both experience them in the same -- or very similar -- way. Age-old philosophical problem solved! But it can't be that simple. So my question is what are the main problems with this "consensus" view of reality? Or, to put...

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!

When the word" exist "occurs like "numbers exist "does it mean what it means in sentences like "Dogs exist"?

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,...

The Kalam Cosmological Argument has as its first premise "Everything that begins to exist has a cause" (at least in the form I've heard it). Often examples of "chairs" or "people" are given for things that began to exist. But this seems to be a category error - the Universe did not begin to exist in the same way that a chair does. Indeed a chair doesn't "begin to exist" in that it was created from other things. So to me it sounds like the argument overstates its case with "everything that begins to exist" since the only thing that has begun to exist is, well, everything. One could restate this premise as "The universe began to exist" could it not? Is I missing something or is this what is meant by this argument? If so it seems to be more of an assumption than the inductive reasoning I hear it being used as (e.g. "you've never seen a chair 'pop into existence' have you?").

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."

Is there a clear way to distinguish physical and non physical things? I'm not implying that there are non physical things. I would prefer if you didn't define "physical" as whatever is studied by physicists.

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 .

If every thing has being then how can non-being, as a concept, not have being? Wouldn't it have being by virtue of its distinction from being? I.e. in being 'non-being'- it 'is' that which is 'not being' (the conceptual complement of being) and as it 'is' non-being - distinct from being, it is 'being' no? And so if this is the case, and being is thus a characteristic of all things - then what of being itself?

...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.

Can philosophy speculate as to the likelihood of their being aliens on other planets or intelligent life elsewhere? If so, what do philosophers have to say about this and what do you philosophers on this forum have to say?

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 .