Random mathematical tangent:
People often say that the real numbers are uncountable. I was told this at at least once at the university, probably several times. I've also read a whole bunch of textbooks without having that corrected.
And yet it's not true. You can't prove whether the real numbers are countable or not. (For those interested, "countable" means "I can specify some fixed way of listing them all one after the other, such that, when you point at any one real number, I always say "ah, that's #3523412 in the list" or something like that.)
The proof goes like this:
1. Take the natural numbers {0, 1, 2, 3, ...}
2. Take the set of all subsets of natural numbers {set of even numbers, set of odd numbers, set of only {1,2}, ...}
3. Prove that this set is uncountable
4. Prove that this set is bijective to the set of real numbers
5. Conclude that the reals are uncountable
Steps 1,2, 4,5 are fine. Step 3 is not. You can't prove that the set of subsets of natural numbers is uncountable.
The argument that purports to show this is called the diagonal lemma. In a nutshell, you take an arbitrary way of listing these subsets and then construct from that a particular subset that's not on the list.
This indeed works. You can construct such a collection. However, that collection may not be a set that exists. This is the problem.
People always point to the power set axiom, one of the axioms of set theory. They think it says that, given any set, all subsets of that set exist and there is a different set that contains them all. Wrong! The power set axiom only says "given a set X, there is a set P such that, for any set S that happens to exist and is a subset of X, that set is in P (and no other sets are in P)". it doesn't say anything about whether or how many such sets P exist. The collection you construct with the diagonal lemma may just not be a set that exists.
In the most common formalization of math, you can't prove that there are unaccountably many sets period. Not in the real numbers and not anywhere else. You can't prove that the set of subsets of real numbers is uncountable. You need second order logic to do that, and while I like that approach, most mathematicians don't think of math as formalized that way.