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By Root 1628 0

easy to define in a recursive fashion, and for that reason wff-ness (exactly like F-ness) is just the kind of mathematical notion that Principia Mathematica was designed to study. To be sure, Whitehead and Russell had never dreamed that their mechanical reasoning system might be put to such a curious use, in which its own properties as a machine were essentially placed under observation by itself, rather like using a microscope to examine some of its own lenses for possible defects. But then, inventions often do surprise their inventors.

Prim Numbers

Having realized that some hypothetical volume of the series by Whitehead and Russell could define and systematically explore the various numerical properties of wff numbers, Gödel pushed his analogy further and showed, with a good deal of fancy machinery but actually not very much conceptual difficulty, that there was an infinitely more interesting recursively defined class of whole numbers, which I shall here call prim numbers (whimsically saluting the title of the famous three tomes), and which are the numbers belonging to provable formulas of PM (i.e., theorems).

A PM proof, of course, is a series of formulas leading from the axioms of PM all the way to the formula in question, each step being allowed by some rule of reasoning, which in PM became a formal typographical rule of inference. To every typographical rule of inference acting on strings of PM, Gödel exhibited a perfectly matching computational rule that acted on numbers. Numerical computation was effectively thumbing its nose at typographical manipulation, sassily saying, “Anything you can do, I can do better!” Well, not really better — but the key point, as Gödel showed beyond any doubt, was that a computational rule would always be able to mimic perfectly — to keep in perfect synchrony with — any formal typographical rule, and so numerical rules were just as good.

The upshot was that to every provable string of Russell and Whitehead’s formal system, there was a counterpart prim number. Any integer that was prim could be decoded into symbols, and the string you got would be a provable-in-PM formula. Likewise, any provable-in-PM formula could be encoded as one whopping huge integer, and by God, with enough calculation, you could show that that number was a prim number. A simple example of a prim number is, once again, our friend 72900, since the formula “0=0”, over and above being a well-formed formula, is also, and not too surprisingly, derivable in PM. (Indeed, if it weren’t, PM would be absolutely pathetic as a mechanical model of mathematical reasoning!)

There is a crucial difference between wff numbers and prim numbers, which comes from the fact that the rules of inference of PM sometimes produce output strings that are shorter than their input strings. This means that the corresponding arithmetical rules defining prim numbers will sometimes take large prim numbers as input and make from them a smaller prim number as output. Therefore, stretches of the number line that have been visited once can always be revisited later, and this fact makes it much, much harder to determine about a given integer whether it is prim or not. This is a central and very deep fact about prim numbers.

Just as with squares, primes, F numbers, or wff numbers, there could once again be a hypothetical volume of the series of tomes by Whitehead and Russell in which prim numbers were defined and their mathematical properties studied. For example, such a volume might contain a proof of the formula of PM that (when examined carefully) asserts “72900 is a prim number”, and it might also discuss another formula that could be seen to assert the opposite (“72900 is not a prim number”), and so on. This latter statement is false, of course, while the former one is true. And even more complex number-theoretical ideas could be expressed using the PM notation and discussed in the hypothetical volume, such as “There are infinitely many prim numbers” — which would be tantamount to asserting (via a code), “There are infinitely many formulas