So many philosophical pseudo-debates focus on the existence or non-existence of this or that “thing”. Pop-skepticism is at odds with most sects of Christianity about the existence of a God; many skeptics, somehow oblivious of the hypocrisy in which they engage, argue simultaneously that claims must be supported with evidence and that there must be no God. I engaged fleetingly with the university’s skeptics society in a debate about the existence of the electron, in which I argued that the electron was a mathematical tool that enjoyed the same level of existence as a number, and not so much existence as…
Fortunately for me, I did not have to complete the above thought, as the debate was on facebook so I was permitted not to respond once the level of abstraction exceeded me. Rather than the inevitable fate of a face-to-face debate on the subject — in which I would make a fool of myself for failing to possess a well-collected, self-consistent argument, my opponents permitting me to exit the arena now having failed to disrupt their conceptual status quo — the debate fizzled out, and they will probably not remember of their own volition that they had even engaged in it. It is all for the better that my medium has changed, since after some time spent meditating on the question, I have come across something I have not been able to distill into a snappy epigram.
To a “standard model” logical mind, and even to the working mathematician who has not studied logic, existence is a straightforward concept. One can ask whether a mathematical object exists with some property, and assume without argument that one is asking a reasonable question with a yes-or-no answer. However, in the world of mathematical logic — the only logical world whose paradoxes I can comfortably resolve — the notion of existence is rather more slippery. There are the standard objects which one can prove to exist from the axioms, and there are — or perhaps I should say, there are not — objects whose existence is contradictory. But there is a neglected middle class. These objects _____ whether or not you choose to exclude the middle.
The Twin Prime Conjecture (TPC), a famous question still open today in 2011, conjectures that there are infinitely many numbers p such that both p and p+2 are prime. One of these pairs is called a “twin prime”, for example 5 and 7, or 179 and 181. There are many who believe TPC is true, some who believe TPC is false, but among logicians (who crave this sort of result), many believe TPC is “independent of the axioms.” Let us explore the consequences of this latter belief. To be concrete (insofar as such a word can mean anything in such matters), let us suppose that TPC is independent of “ZFC”, the Zermelo Frankel axioms with the Axiom of Choice, the axioms of choice (no pun intended) for popular set theory.
It would be helpful to be reminded of what exactly ZFC is. Aside from the deep fantastic worlds of intuition inhabiting many mathematicians’ minds, it is merely a set of 9 statements about the world of sets. For example, “if two sets have the same members, then they are the same set”, and “given any set, you may form the subset of elements satisfying a particular property”. These are stated in rigorous, precise logical language, so by formal manipulation we can exclude the subtleties of meaning that would abound in any English presentation of these axioms. Logicians like to say that a proof is nothing more than a chain of formal logical sentences arranged according to some simple rules; this view has spread since the advent of programming languages and computerized mathematical assistants.
If TPC were true, then given any number, you could count up from that number and eventually reach a twin prime. If TPC were false, then there would be some number, call it L, above which it would not be possible to find any twin primes. However, since TPC is independent (because we have supposed it), then we know we cannot prove it either way. It may be true, or it may be false; whether there is a third option is too deep a philosophical question to explore here. We may be able to count up from any number and find a twin prime, but we will never be sure that we will not arrive at a point after which there are no more. Or there may in fact be an L above which there are no more, but we shall never be able to write L as a sequence of digits. Again, whether these two comprise all possibilities is not a matter capable of absolute resolution.
There can be no proof that L exists, so, like God to the skeptics, it must not exist. By their own standard, this conclusion is not justified, for, by our assumption, there is no evidence in favor of its nonexistence either. Indeed, we may safely believe in L; if a contradiction would arise from its use, then we could leverage that contradiction to provide a proof that there are infinitely many twin primes, thus TPC would have been provable. After centuries of cautious hypothesis of what would happen if L did exist, we may begin to treat L as any other number. As the ancient Greeks’ unease about the existence of irrational numbers has faded, so too would ours. The naturals would become: 1, 2, 3, 4, 5, … L, L+1, …. We will have answered questions about L, for example it is greater than one million, because have found twin primes greater than one million.
This all happens consistently with the proof that the set of natural numbers is made up of only the numbers 1, 2, 3, 4, 5, …, for that proof does not mean what we think it means. We cannot enumerate all the natural numbers in a theorem; that proof only states that the set of natural numbers is the smallest set made up of zero and successors of elements in that set. If we can actually find a twin prime above any number, but merely not know it, then we might claim L cannot be the successor of any element in this set. But this claim is false, because L is clearly the successor of L-1! L, whether or not or ___ it is one of the familiar numbers, manages to sneak its way into the smallest set containing zero and successors. It is not the set of numbers, but the language about numbers that can be extended by this independence of TPC, and L is not logically distinguishable from “regular” numbers. It is a symbolic phenomenon. But so, too, are the familiar numbers. The only difference is we have chosen to say that zero exists.