# What is a paradox in mathematics?

I was reading some of these "mathematical paradoxes", and trying to understand why the list presents only counterintuitive mathematical results.

Is there room in mathematics for logical paradoxes?

-

A paradox is a true result that is surprising to our human sensibilities. These are the kinds of things on the list you provide. There is nothing wrong with paradoxes of this sort. Indeed, having our intuition turned on its head is (in my opinion) one of the great things about mathematics.

A contradiction would arise in a logical system that purports two opposite statements to be true (for example, a model of arithmetic in which you can derive both $1 + 1 = 2$ and $1 + 1 \neq 2$). Logical contradictions are not permissible in mathematics, since one can derive any statement (true or false) from a contradiction.

-
I say that paradoxes must be true statements, but this is not always how the word is used. Russell's Paradox, for example, would be more rightly called Russell's Contradiction in my terminology. Perhaps it is a historical accident that it is called a paradox (or else I am being to strict in my use of the word). –  Austin Mohr Jan 1 at 22:25
I think your understanding of "paradox" is too limited and is at odds with the way the word has been used historically. W.V.O. Quine identifies several sorts of paradoxes. One he calls a "veridical" paradox, which is an absurd-seeming claim that is nevertheless true; the Banach-Tarski paradox is of this type. A "falsidical" paradox is a statement arrived at by seemingly correct logic that is nevertheless clearly false; Zeno's paradoxes are of this sort. –  MJD Jan 1 at 23:41
@MJD I am inclined to agree that my use of the word here is too limited. Still, I think OP's confusion arose from the overloading of the word to refer both to counterintuitive results and logical contradictions. –  Austin Mohr Jan 2 at 0:58

There are two meanings in which one can interpret "paradox" when talking about mathematics:

1. Contradiction which is naturally derived from assumptions, e.g. Russell's paradox, or Cantor's paradox. These are sentences which exhibit the inconsistency of a definition. For example Burali-Forti shows that there is no set which contains "all the ordinals". Therefore any system of definitions in which every collection is a set will be inconsistent (granted we can define an ordinal in this system, which is something we expect to be able to do in a reasonable set theory).

2. Counterintuitive results which show how strange mathematics can be. The best known example is the Banach-Tarski paradox which states that assuming the axiom of choice (and in fact a very weak variant of it) we can prove the the unit ball in $\mathbb R^3$ can be partitioned into five pieces and recomposed as two balls whose volume is twice the original.

Much less known is the paradox arises from assuming all sets are Lebesgue measurable (in ZF+DC). In such model we can partition the real numbers into more parts than sets. Yes, we can cut a set into a strictly greater number of parts than we have elements to partition. Sounds strange? Well, this is just one of the things that can go wrong when not assuming the axiom of choice!

-

In classical logic logical paradoxes are not allowed but in paraconsistent logic they are allowed.

There are several types of paradox. There is one that we may call a phenomenological paradox, one where the mathematical results contradict basic truths about what the mathematics is supposed to model. For example, The Banach-Tarski paradox can be considered such a paradox. Then there are logical paradoxes, that is a statement that is provably both true and false.

In classical logic (where every statement is either true or false, but not both) it can easily be shown that a logical contradiction entails every other statement and thus if a logical contradiction exists in a logical system then that system is quite useless. Therefore, paradoxes must be banished. Phenomenological paradoxes are banished by either fine-tuning the axioms so that the paradoxical result no longer follows or by accepting the result as true and announce that our intuition has been refined. Logical paradoxes are usually resolved by carefully fine-tuning the axioms or by somehow disallowing the paradoxical creatures away from the discussion.

In paraconsistent logic, where a statement may, without causing any logical maladies, be both true and false (see http://plus.maths.org/content/not-carrot for a nice introduction) logical paradoxes are quite different and can, sometimes, be harmlessly accepted.

-

No, there isn't. Within sound mathematics, one cannot obtain inconsiatnt results. Then again, we simply cannot prove consistency of mathematics (and it has been proven that we cannot, at least not if it is true). Thus - paradoxically - yes, there is room in mathematics for logical paradoxes, though it is just some tiny little corner and we know there is none, but we cannot prove that.

-
+1 I would like to find more information about "consistency of mathematics", and the lack of a proof –  rraallvv Jan 1 at 22:04
@rraallvv You'll find Godel's incompleteness theorems interesting. –  Austin Mohr Jan 1 at 22:06
@AustinMohr If I've got your point, mathematics cannot be used to prove consistency of mathematics... interesting! –  rraallvv Jan 1 at 22:14

You can quibble about what precisely is meant by the term "paradox," but if I understand your meaning, the so-called liar paradoxes can be seen as logical paradoxes. The original liar paradox, for example, is based on a claim by the Cretan poet, Epimenides (circa 600 BC), that Cretans are "always liars." Some will argue, using what I think is an overly strict interpretation, that this is not a "true paradox" since it can be resolved by simply pointing out that Epimenides' claim must itself have been a lie, and that, therefore, at least one Cretan must once have told the truth. (One logically consistent scenario that satisfies this requirement would be that all Cretans but Epimenides himself always told the truth!)

-