What does it mean when a certain axiom is neither provable nor deniable? Does it imply that such axiom is self-evident and can't be doubted? I don't think that "real skeptics"(a skeptic who is so deep in doubt that he doubts his own existence and even his own doubt) like Pyrrho would be happy with that.

Let's consider, for example, what philosopher Hilary Putnam has called "the minimal principle of contradiction": (MPC) Not every contradiction is true. Arguably, MPC is unprovable because whichever premises and inference rules we might use to try to prove MPC are no better-known by us, and no more securely correct, than MPC itself is. But MPC would also appear to be undeniable, since in standard logic to deny MPC is to imply that every contradiction is true, and it's hard (for me, anyway) to make any sense of the notion of denying something in circumstances in which every contradiction is true. So, arguably, MPC is self-evident and can't be doubted: that is, the notion of MPC' s being doubted makes no sense. You suggest that this result would bother...

Are mathematical truths such as 2+2 =4 arguable exceptions to the correspondence theory of truth? I mean is 2+2=4 a truth that corresponds to "the world"?

I don't think mathematical truths pose a special problem for the correspondence theory of truth (see this link for more about the theory). The correspondence theorist can interpret "the world" broadly enough to include abstract objects, aspects of mathematical reality, and so on. In other words, "the world" needn't be restricted to the physical universe.

Are 3 and √9 the same mathematical object (in light of the fact that they have the same numerical value), or are they distinct mathematical objects? In other words, are the expressions '3' and '√9' co-referential names (both referring to the number 3), or do they refer to different objects?

If "√9" refers to the positive square root of 9 (I'm not sure what the convention is concerning the square-root symbol), then I'd say that 3 and √9 are the same object, just as Mark Twain and Samuel Clemens are the same object. (Indeed, the plural verb "are" in each case is a bit of loose talk.) Leibniz's Law (the Indiscernibility of Identicals) therefore implies that everything true of 3 is true of √9, and everything true of Twain is true of Clemens, which seems right to me.

Hi, I was hoping for some clarification from Professor Maitzen about his comments on infinite sets (on March 7). The fact that every natural number has a successor is only true for the natural numbers so far encountered (and imagined, I suppose). Granted, I can't conceive of how it could be that we couldn't just add 1 to any natural number to get another one, but that doesn't mean it's impossible. It seems quite strange, but there are some professional mathematicians who claim that the existence of a largest natural number (probably so large that we would never come close to dealing with it) is much less strange and problematic than many of the conclusions that result from the acceptance of infinities. If we want to define natural numbers such that each natural number by definition has a successor, then mathematical induction tells us there are infinitely many of them. But mathematical induction itself only proves things given certain mathematical definitions. Whether those definitions indeed...

Thanks for sending a follow-up question. Prof. Heck, who knows this territory better than I do, provided helpful corrections and amplifications in his answer to Question 5068 . I recommend taking another look there. You wrote, "The fact that every natural number has a successor is only true for the natural numbers so far encountered (and imagined, I suppose)." The claim that every natural number has a unique successor, like the claim that 1 isn't the successor of any natural number, is an axiom -- a starting point -- rather than a conclusion drawn from examining or imagining data. Your fifth sentence suggests that you know this already. You're quite right that math induction proves its results only given its starting points, but of course that's true of all proofs: all proofs (even proofs that have no premises) rely on essential assumptions. You say that some professional mathematicians would rather accept the existence of a largest natural number than accept the paradoxical features of...

Do infinite sets exist? Most mathematicians say yes, but to me it seems like infinite sets can only exist if we use inductive reasoning but not deductive reasoning. For example, in the set {1,2,3,4,...} we can't prove that the ... really means what we want it to. No one has shown that the universe doesn't implode before certain large enough "numbers" are ever glimpsed, so how can we say they exist as part of an "object" like a set. We can only do this by assuming the existence of the rest of the set since that seems logical base on our experience. But that seems like a rather weak argument.

We can use mathematical induction to prove that (i) infinitely many natural numbers exist from the premise that (ii) 1 is a natural number and the premise that (iii) every natural number has a successor. Although it's called mathematical "induction," it's actually deductive reasoning. I take it that (ii) is beyond dispute, and (iii) is at any rate very hard to deny! It won't do to demand proof of (ii) or (iii) before accepting this proof of (i), for if the premises in any proof must themselves have been proven, then we have an infinite regress: nothing could be proven in a finite amount of time. We've therefore proven that infinitely many natural numbers exist. The notation "{1,2,3,4,...}" is just one way of referring to the set containing all and only those infinitely many numbers. It's perhaps a fallible way of referring to that set, because it assumes that the audience knows which number comes next in the series. A more reliable way of referring to the set is "the set whose members are the...

Euclid in "Elements" wrote that "things which equal the same thing also equal one another." Is this true in all cases? I've read that it is only true for "absolute entities," but not to "relations," although I do not understand this exemption. Are there any examples of things that are equal to the same thing but not to one another? Are relations really exempt from Euclid's axiom, and if so, why?

If by the adjective "equal" Euclid means "identical in magnitude" (which I gather is what he does mean), then his principle follows from the combination of the symmetry of identity and the transitivity of identity . The symmetry of identity says that, for any x and y , x is identical to y if and only if y is identical to x . The transitivity of identity says that, for any x , y , and z , if x is identical to y and y is identical to z , then x is identical to z . Therefore, Euclid's principle has exceptions only if the symmetry of identity sometimes fails or the transitivity of identity sometimes fails. But I don't think either of them ever fails. Now, some relations that are similar to the identity relation aren't transitive. I might be (1) unable to tell the difference between color swatches A and B, (2) unable to tell the difference between swatches B and C, yet (3) able to tell the difference between swatches A and C. But...

I've recently read that some mathematician's believe that there are "no necessary truths" in mathematics. Is this true? And if it is, what implications would it have on deductive logic, it being the case that deductive logical forms depend on mathematical arguments to some degree. Would in this case, mathematical truths be "contingently-necessary"?

Your question is tantalizing. I wish it had included a citation to mathematicians who say what you report them as saying. On the face of it, their claim looks implausible. Are there no necessary truths at all? If there are necessary truths, how could the mathematical truth that 1 = 1 not be among them? One way to hold that mathematicians seek only contingent truths might be as follows. If some philosophers are correct that propositions are to be identified with sets of possible worlds, then there's only one necessarily true proposition, because there's only one set whose members are all the possible worlds there are. That single necessarily true proposition (call it "T") will be expressed by indefinitely many different sentences , including the sentences "1 = 1" and "No red things are colorless," and it will be contingent just which sentences express T. On this view, mathematicians don't try to discover various necessary truths, since there's just one necessary truth, T. ...

Is it possible for a mathematical equation to both be fundamentally unsolvable and also have a correct answer?

I hope philosophers of math on the Panel will respond with more authority than I have. My understanding is that G ö del showed that arithmetic contains pairs of mutually contradictory statements neither one of which is provable within arithmetic. Assuming the standard logical law that exactly one of every pair of mutually contradictory statements is true, we get the result that some arithmetical truths are unprovable within arithmetic. I can't say whether those truths include statements to the effect that such-and-such is the solution to an equation, but if they do, and if their being unprovable within arithmetic makes the associated equations "fundamentally unsolvable," then the answer to your question is yes . Someone might reply that an unprovable arithmetical statement can't be true , but I think that would be to mistake truth for provability.

Is mathematics grounded in logic or is logic grounded in mathematics?

I leave it to the experts on the Panel (and there are several) to give you a proper answer, but I would certainly reject the second of your alternatives: I can't see how logic could be grounded in mathematics. It's a more controversial issue whether mathematics is grounded in logic and, if it is, what that grounding amounts to.

The equality x-x=0 and 0=x-x are suppose to be the same. The first equality is easy to understand while the second equality( 0=x-x )is somewhat mind boggling to me for the following reason: where do the 2x's on the right side come from? Thanks Kal

Assuming I understand your question: They come from the same "place" in each equation, namely, from anywhere at all. It might help to think of it this way: "What's the result of subtracting any magnitude at all from itself? Zero." "What's zero? The result of subtracting any magnitude at all from itself." Each answer is just as good as the other in answering the respective question being asked.