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I've been interested in this question for some time. I haven't put any serious thought into it, so all I can offer is a further question rather than an answer. (I'm interested in the answers that have already been given though.) My question is this. Is there a system of logic that will allow us to prove only statements that have physical meaning? I don't have a formal definition of "physically meaningful" so instead let me try to illustrate what I mean by an example or two.

Consider first the statement that the square root of 2 is irrational. What would be its physical meaning? A naive suggestion would be that if you drew an enormous grid of squares of side length one centimetre and then measured the distance between (0,0) and (n,n) for some n, then the result would never be an integer number of centimetres. But this isn't physically meaningful according to my nonexistent definition because you can't measure to infinite accuracy. However, the more finitistic statement that the square root of 2 can't be well approximated by irrationals has at least some meaning: it tells us that if n isn't too large then there will be an appreciable difference between the distance from (0,0) to (n,n) and the nearest integer.

As a second example, take the statement that the sum of the first n positive integers is n(n+1)/2. If n is too huge, then there is no hope of arranging a huge triangular array and counting how many points are in it. So one can't check this result experimentally once n is above a certain threshold (though there might be ingenious ways of checking it that are better than the obvious method). This shows that we can't apply unconstrained induction, but there could be a principle that said something like, "If you keep on going for as long as is practical, then the result will always hold."

One attitude one might take is that this would be to normal classical mathematics as the use of epsilons and deltas is to the mathematics of infinities and infinitesimals. One could try to argue that statements that appear to be about arbitrarily large integers or arbitrarily small real numbers (or indeed any real numbers to an arbitrary accuracy) are really idealizations that are a convenient way of talking about very large integers, very small real numbers and very accurate measurements.

If I try to develop this kind of idea I rapidly run into difficulties. For example, what is the status of the argument that proves that the sum of the first n integers is what it is because you can pair them off in a nice way? In general, if we have a classical proof that something will be the case for every n, what do we gain from saying (in some other system) that the conclusion of the proof holds only for every "feasible" n? Why not just say that the classical result is valid, and that this implies that all its "feasible manifestations" are valid.?

Rather than continue with these amateur thoughts, I'd just like to ask whether similar ideas are out there in a better form. Incidentally, I'm not too fond of Zeilberger's proposal because he has a very arbitrary cutoff for the highest integer -- I'd prefer something that gets fuzzier as you get larger.

Edit: on looking at the Sazonov paper, I see that many of these thoughts are in the introduction, so that is probably a pretty good answer to my question. I'll see whether I find what he does satisfactory.

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I've been interested in this question for some time. I haven't put any serious thought into it, so all I can offer is a further question rather than an answer. (I'm interested in the answers that have already been given though.) My question is this. Is there a system of logic that will allow us to prove only statements that have physical meaning? I don't have a formal definition of "physically meaningful" so instead let me try to illustrate what I mean by an example or two.

Consider first the statement that the square root of 2 is irrational. What would be its physical meaning? A naive suggestion would be that if you drew an enormous grid of squares of side length one centimetre and then measured the distance between (0,0) and (n,n) for some n, then the result would never be an integer number of centimetres. But this isn't physically meaningful according to my nonexistent definition because you can't measure to infinite accuracy. However, the more finitistic statement that the square root of 2 can't be well approximated by irrationals has at least some meaning: it tells us that if n isn't too large then there will be an appreciable difference between the distance from (0,0) to (n,n) and the nearest integer.

As a second example, take the statement that the sum of the first n positive integers is n(n+1)/2. If n is too huge, then there is no hope of arranging a huge triangular array and counting how many points are in it. So one can't check this result experimentally once n is above a certain threshold (though there might be ingenious ways of checking it that are better than the obvious method). This shows that we can't apply unconstrained induction, but there could be a principle that said something like, "If you keep on going for as long as is practical, then the result will always hold."

One attitude one might take is that this would be to normal classical mathematics as the use of epsilons and deltas is to the mathematics of infinities and infinitesimals. One could try to argue that statements that appear to be about arbitrarily large integers or arbitrarily small real numbers (or indeed any real numbers to an arbitrary accuracy) are really idealizations that are a convenient way of talking about very large integers, very small real numbers and very accurate measurements.

If I try to develop this kind of idea I rapidly run into difficulties. For example, what is the status of the argument that proves that the sum of the first n integers is what it is because you can pair them off in a nice way? In general, if we have a classical proof that something will be the case for every n, what do we gain from saying (in some other system) that the conclusion of the proof holds only for every "feasible" n? Why not just say that the classical result is valid, and that this implies that all its "feasible manifestations" are valid.

Rather than continue with these amateur thoughts, I'd just like to ask whether similar ideas are out there in a better form. Incidentally, I'm not too fond of Zeilberger's proposal because he has a very arbitrary cutoff for the highest integer -- I'd prefer something that gets fuzzier as you get larger.

Edit: on looking at the Sazonov paper, I see that many of these thoughts are in the introduction, so that is probably a pretty good answer to my question. I'll see whether I find what he does satisfactory.