From your post it sounds like you in fact do not have a clear picture of infinity in your head. I have a feeling this is true for many people, so let me try to paint one. Throughout this post I’ll be using “number” to mean “positive integer”.
Suppose that there is a distinction we can draw between certain types of numbers and other types of numbers. For example, we could make a distinction between “primes” and “non-primes”. A standard way to communicate the fact that we have drawn this distinction is to say that there is a “set of all primes”. This language need not be construed as meaning that all primes together can be coherently thought of as forming a collection (though it often is construed that way, usually pretty carelessly); the key thing is just that the distinction between primes and non-primes is itself meaningful. In the case of primes, the fact that the distinction is meaningful follows from the fact that there is an algorithm to decide whether any given number is prime.
Now for “infinite”: A set of numbers is called infinite if for every number N, there exists a number greater than N in the set. For example, Euclid proved that the set of primes is infinite under this definition.
Now this definition is a little restrictive in terms of mathematical practice, since we will often want to talk about sets that contain things other than numbers, but the basic idea is similar in the general case: the semantic function of a set is provided not by the fact that its members “form a collection” (whatever that might mean), but rather by the fact that there is a distinction of some kind (possibly of the kind that can be determined by an algorithm) between things that are in the set and things that are not in the set. In general a set is “infinite” if for every number N, the set contains more than N members (i.e. there are more than N things that satisfy the condition that the set encodes).
So that’s “infinity”, as used in standard mathematical practice. (Well, there’s also a notion of “infinity” in real analysis which essentially is just a placeholder symbol for “a really large number”, but when people talk about the philosophical issues behind infinity it is usually about the definition I just gave above, not the one in real analysis, which is not controversial.) Now, why is this at all controversial? Well, note that to define it, I had to talk about the notion of distinctions-in-general, as opposed to any individual distinction. But is it really coherent to talk about a notion of distinctions-in-general? Can it be made mathematically precise? This is really what the philosophical arguments are all about: what kinds of things are allowed to count as distinctions. The constructivists take the point of view that the only things that should be allowed to count as distinctions are those that can be computed by algorithms. There are some bullets to bite if you take this point of view though. For example, the twin prime conjecture states that for every number N, there exists p > N such that both p and p+2 are prime. Presumably this is either true or false, even if nobody can prove it. Moreover, presumably each number N either is or is not a counterexample to the conjecture. But then it would seem that it is possible to draw a distinction between those N which satisfy the conclusion of the conjecture, and those which are counterexamples. Yet this is false according to the constructive point of view, since there is no algorithm to determine whether any given N satisfies the conclusion of the conjecture.
I guess this is probably long enough already given that I’m replying to a five-year-old post… I could say more on this topic if people are interested.
I think my original sentence is correct; there is no known algorithm that provably outputs the answer to the question “Does N satisfy the conclusion of the conjecture?” given N as an input. To do this, an algorithm would need to do both of the following: output “Yes” if and only if N satisfies the conclusion, and output “No” if and only if N does not satisfy the conclusion. There are known algorithms that do the first but not the second (unless the twin prime conjecture happens to be true).
From your post it sounds like you in fact do not have a clear picture of infinity in your head. I have a feeling this is true for many people, so let me try to paint one. Throughout this post I’ll be using “number” to mean “positive integer”.
Suppose that there is a distinction we can draw between certain types of numbers and other types of numbers. For example, we could make a distinction between “primes” and “non-primes”. A standard way to communicate the fact that we have drawn this distinction is to say that there is a “set of all primes”. This language need not be construed as meaning that all primes together can be coherently thought of as forming a collection (though it often is construed that way, usually pretty carelessly); the key thing is just that the distinction between primes and non-primes is itself meaningful. In the case of primes, the fact that the distinction is meaningful follows from the fact that there is an algorithm to decide whether any given number is prime.
Now for “infinite”: A set of numbers is called infinite if for every number N, there exists a number greater than N in the set. For example, Euclid proved that the set of primes is infinite under this definition.
Now this definition is a little restrictive in terms of mathematical practice, since we will often want to talk about sets that contain things other than numbers, but the basic idea is similar in the general case: the semantic function of a set is provided not by the fact that its members “form a collection” (whatever that might mean), but rather by the fact that there is a distinction of some kind (possibly of the kind that can be determined by an algorithm) between things that are in the set and things that are not in the set. In general a set is “infinite” if for every number N, the set contains more than N members (i.e. there are more than N things that satisfy the condition that the set encodes).
So that’s “infinity”, as used in standard mathematical practice. (Well, there’s also a notion of “infinity” in real analysis which essentially is just a placeholder symbol for “a really large number”, but when people talk about the philosophical issues behind infinity it is usually about the definition I just gave above, not the one in real analysis, which is not controversial.) Now, why is this at all controversial? Well, note that to define it, I had to talk about the notion of distinctions-in-general, as opposed to any individual distinction. But is it really coherent to talk about a notion of distinctions-in-general? Can it be made mathematically precise? This is really what the philosophical arguments are all about: what kinds of things are allowed to count as distinctions. The constructivists take the point of view that the only things that should be allowed to count as distinctions are those that can be computed by algorithms. There are some bullets to bite if you take this point of view though. For example, the twin prime conjecture states that for every number N, there exists p > N such that both p and p+2 are prime. Presumably this is either true or false, even if nobody can prove it. Moreover, presumably each number N either is or is not a counterexample to the conjecture. But then it would seem that it is possible to draw a distinction between those N which satisfy the conclusion of the conjecture, and those which are counterexamples. Yet this is false according to the constructive point of view, since there is no algorithm to determine whether any given N satisfies the conclusion of the conjecture.
I guess this is probably long enough already given that I’m replying to a five-year-old post… I could say more on this topic if people are interested.
I think you mean, ‘determine that it does not satisfy the conclusion’.
I think my original sentence is correct; there is no known algorithm that provably outputs the answer to the question “Does N satisfy the conclusion of the conjecture?” given N as an input. To do this, an algorithm would need to do both of the following: output “Yes” if and only if N satisfies the conclusion, and output “No” if and only if N does not satisfy the conclusion. There are known algorithms that do the first but not the second (unless the twin prime conjecture happens to be true).