At least 33, to judge from the Areas of Specialization (AOS) listed here: http://philpapers.org/surveys/results.pl. Less facetiously: There's no non-arbitrary way to give a precise answer to your question, just as with the question "How many branches of science are there?" Careful attention to language is essential because, among other things, only when we're careful about language can we tell which philosophical problem (if any) we're trying to solve or which philosophical question (if any) we're trying to answer.

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...

To start with the literal truth: Men can indeed speak about issues pertaining to women, because (some) men do speak about such issues, which proves that they can . The question is how authoritative, how credible, about those issues a man can be. I think that will depend on the issue. An appropriately trained male researcher can speak with authority about (for example) the long-term health effects of this or that means of female contraception. But if the issue is more "experiential" or "phenomenological" -- such as "What does it feel like to be a female victim of workplace sexual harassment?" -- then it seems clear that any male has at least a burden of proof to discharge before we should regard his statements as authoritative or credible. I'm not saying that this burden can't possibly be discharged, only that it exists. (Clearly, I assume that speaking credibly about whether a man can be authoritative on issue X doesn't require me to be an authority on issue X itself.) For my answer to a...

Your questions touch on several interesting and difficult issues. If you haven't already consulted it, I recommend looking at the SEP entry on "Possible Objects" . It's a somewhat challenging read, but it contains discussion (for example, in Section 2.2) of the issues you raised, and (as usual for the SEP) copious references that you can follow up. Best of luck as you grapple with this topic!

The argument itself is logically valid -- indeed, formally valid. It uses only modus tollens and the rule that "P" and "~ ~ P" are equivalent, both of which are valid rules of inference. However, I think the argument is unsound -- and therefore I think it fails to establish its conclusion -- because Premise 1 is false, at least if Premise 1 is meant as a strict conditional. (I think it's also false if it's meant as a material conditional, but that's more controversial.) For excellent discussion of Premise 1, I recommend this article and this collection of essays .

It's wonderful to hear that at the age of 12 you've already read a lot, and I'm impressed by your submission. Many people twice your age wouldn't be able to submit something so thoughtful. I would recommend that you take a look at the essays in John Martin Fischer's edited volume on the metaphysics of death . I found it helpful when I read it many years ago; it contains both tough-minded philosophy and also bits of comic relief. You may find it a challenging read, but I don't think it's beyond you. Nowadays there may be more up-to-date collections on the topic, but Fischer's volume is an excellent place to start.

I'd say yes . One way would be if (for whatever reason) you ceased to believe some proposition P that you formerly knew to be true. If belief is a precondition for knowledge, then you'd no longer know that P. Another way might be, while retaining your belief of P, to come to believe (or indeed even know) some proposition Q your belief of which undermines your justification for believing P and thereby deprives you of the knowledge of P that you formerly had. See Carl Ginet's article "Knowing Less By Knowing More" (linked here ).

I myself would answer "Yes" to your final question. I think you've put your finger on what distinguishes good philosophers from good practitioners of other disciplines: the desire and the ability to attain the highest standards of argumentative attentiveness and rigor regardless of the topic at hand. We can rely on good mathematicians, physicists, and biologists (for example) to be careful and rigorous about math, physics, and biology. But get them outside their scientific specialties, and the results are very much hit-or-miss, with a lot of miss: Richard Feynman, Stephen Hawking, Neil deGrasse Tyson, Lawrence Krauss, Jerry Coyne, and Bill Nye on the nature and value of philosophy; Sam Harris on free will and on the ability of science to resolve ethical questions; Coyne on free will; Richard Dawkins on free will and on the sorites paradox; Krauss on why there's something rather than nothing. Those are examples of sloppy argumentation that come readily to mind from just my own reading. It's not...

I'm intrigued by your suggestion that there may be a paradox here, but I'm having trouble reconstructing your reasoning. As far as I can tell, your reasoning relies on the premise that any definition must contain whatever it defines . But I'm not sure that premise is plausible. Merriam-Webster.com defines "walrus" as follows: "a large gregarious marine mammal ( Odobenus rosmarus of the family Odobenidae) of arctic waters that is related to the seals and has long ivory tusks, a tough wrinkled hide, and stiff whiskers and that feeds mainly on bivalve mollusks." That definition (more precisely, the definiens ) doesn't contain the word ("walrus") being defined, which is good: otherwise the definition would be unhelpful. Nor does the definition contain a walrus; we'd need a cage to do that. Instead, the definition gives us a string of words meant to pick out the walrus from among other things. If it's a genuine definition, then the principle on which it seems you're relying is false. But I may...

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 .