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Why can't we just claim a paradox invalid, just as the way we treat contradiction in mathematical proof? (i.e. if we arrive at the proposition inconsistent with the assumption, then we can immediately claim the logical impossibility of the assumption).

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I will add to Hunan answer only the SEP entry regarding Paradoxes and Contemporary Logic. –  Mauro ALLEGRANZA Mar 30 '14 at 14:57
Given that an argument usually relies on more than one assumption, the difficulty is to tease out which assumption is problematic. –  Colin Tan Mar 30 '14 at 18:05
Do you regard the Banach-Tarski paradox as a paradox? Why or why not? If not, then what is your definition of a paradox? If so, then are you claiming it should be "claimed invalid?" Which means what, exactly? –  user4894 Mar 30 '14 at 19:28
@user4894 No, the Banach-Tarski paradox isn't a paradox: it's just a surprising consequence of a particular set of axioms. Most "paradoxes" in maths and physics are of this form. –  David Richerby Mar 30 '14 at 21:04

2 Answers 2

Paradoxes are indeed invalid arguments, but what makes them special is that they rest on seemingly unproblematic assumptions. We know, for example, that Achilles will in fact outrun the tortoise (Zeno's Paradoxes), we know that the surprise exam will take place (Surprise Exam Paradox), and so on. Because we know the conclusions of those paradoxes are false, we know that something is wrong with the arguments. The task then is to identify the assumptions that lead to the false conclusion.

Paradoxes can be called 'invalid' and ignored, but if taken seriously they can help us diagnose and fix problems with existing logico-mathematical frameworks. Axiomatic set theory and type theory owe much to Russell's Paradox, for example. The above mentioned Surprise Exam Paradox has led to lots of interesting developments in epistemic, dynamic, and public announcement logics. There are, of course, the classical ones, like the Sorites, the Liar, and so on. Each has opened some interesting door.

I claimed that paradoxes are invalid arguments. Sequitur's contribution inspired me to add that someone might ask: "invalid according to which logic?" I'd say classical bivalent first-order, but there are possibilities for significant 'paradox-preserving' deviations from that. What's important here is to realize that: you can't simply change the logic and claim that the paradox is resolved. Suppose classical logic C gives rise to paradox Π, but intuitionistic logic I does not. You cannot simply dispose of classical logic, adopt an intuitionistic one, and claim that you have handled the paradox. Even after doing that, fact will remain that Π is a paradox for C!, so one needs, if interested, to address why C enables the paradox.

Take paradoxes seriously, because they indicate that (at least) some thing is not as true as it seems.

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Just to adress your question directly: We could treat paradoxes in the way suggested by you if the English predicates of truth and falsity were necessarily exclusive; that is if contradictions could not be true.

But whether this is so is a highly controversial issue. Proponents of weak paraconsistency say no, because (i) the English consequence relation is not explosive, i.e. contradictions do not entail everything (think of inconsistent fictions or theories, where apparently not everything holds) and (ii) consequence is truth preservation in all models (of a suitable non-classical logic) and (iii) models represent possibilities.

More boldly, propopnents of strong paraconsistency (aka dialetheism) hold that there actually are true contradictions and so true falsities are possible. Indeed dialetheists say that some paradoxes (such as the liar) are sound arguments. Non-trivial strong paraconsistency entails weak paraconsistency, but not the other way round.

If any of these positions can succeed depends on many intricate issues such as how best to treat paradoxes of self-reference (a strong justification for strong paraconsistency) and a quick settlement of the debate is not to be expected. So you're not justified in treating paradoxes by denying their validity.

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