destruction were already hinted at by the seemingly innocent fact that PM had enough power to talk about arbitrarily subtle properties of whole numbers.
People of earlier eras had intuited much of this richness when they had tried to embed the nature of many diverse aspects of the world around us — stars, planets, atoms, molecules, colors, curves, notes, harmonies, melodies, and so forth — in numerical equations or other types of numerical patterns. Four centuries ago, launching this whole tendency, Galileo Galilei had famously declared, “The book of Nature is written in the language of mathematics” (a thought that must seem shocking to people who love nature but hate mathematics). And yet, despite all these centuries of highly successful mathematizations of various aspects of the world, no one before Godel had realized that one of the domains that mathematics can model is the doing of mathematics itself.
The bottom line, then, is that the unanticipated self-referential twist that Godel found lurking inside Principia Mathematica was a natural and inevitable outcome of the deep representational power of whole numbers. Just as it is no miracle that a video system can create a self-referential loop, but rather a kind of obvious triviality due to the power of TV cameras (or, to put it more precisely, the immensely rich representational power of very large arrays of pixels), so too it is no miracle that Principia Mathematica (or any other comparable system) contains self-focused sentences like Godel’s formula, for the system of integers, exactly like a TV camera (only more so!), can “point” at any system whatsoever and can reproduce that system’s patterns perfectly on the metaphorical “screen” constituted by its set of theorems. And just as in video feedback, the swirls that result from PM pointing at itself have all sorts of unexpected, emergent properties that require a brand-new vocabulary to describe them.
CHAPTER 12
On Downward Causality
Bertrand Russell’s Worst Nightmare
TO MY mind, the most unexpected emergent phenomenon to come out of Kurt Godel’s 1931 work is a bizarre new type of mathematical causality (if I can use that unusual term). I have never seen his discovery cast in this light by other commentators, so what follows is a personal interpretation. To explain my viewpoint, I have to go back to Godel’s celebrated formula — let’s call it “KG” in his honor — and analyze what its existence implies for PM.
As we saw at the end of Chapter 10, KG’s meaning (or more precisely, its secondary meaning — its higher-level, non-numerical, non-Russellian meaning, as revealed by Godel’s ingenious mapping), when boiled down to its essence, is the whiplash-like statement “KG is unprovable inside PM.” And so a natural question — the natural question — is, “Well then, is KG indeed unprovable inside PM?”
To answer this question, we have to rely on one article of faith, which is that anything provable inside PM is a true statement (or, turning this around, that nothing false is provable in PM). This happy state of affairs is what we called, in Chapter 10, “consistency”. Were PM not consistent, then it would prove falsities galore about whole numbers, because the instant that you’ve proven any particular falsity (such as “0=1”), then an infinite number of others (“1=2”, “0=2”, “1+1=1”, “1+1=3”, “2+2=5”, and so forth) follow from it by the rules of PM. Actually, it’s worse than that: if any false statement, no matter how obscure or recondite it was, were provable in PM, then every conceivable arithmetical statement, whether true or false, would become provable, and the whole grand edifice would come tumbling down in a pitiful shambles. In short, the provability of even one falsity would mean that PM had nothing to do with arithmetical truth at all.
What, then, would Bertrand Russell’s worst nightmare be? It would be that someday, someone would come up with a PM proof of a formula expressing an untrue arithmetical statement (“0 = s0” is a good example), because the moment that that happened, PM would be fit for the dumpster. Luckily for Russell, however, every logician on earth would give you better odds for a snowball’s surviving a century in hell. In other words, Bertrand Russell’s worst nightmare is truly just a nightmare, and it will never take place outside of dreamland.
Why would logicians and mathematicians — not just Russell but all of them (including Godel) — give such good odds for this? Well, the axioms of PM are certainly true, and its rules of inference are as simple and as rock-solidly sane as anything one could imagine. How can you get falsities out of that? To think that PM might have false theorems is, quite literally, as hard as thinking that two plus two is five. And so, along with all mathematicians and logicans, let’s give Russell and Whitehead the benefit of the doubt and presume that their grand palace of logic is consistent. From here on out, then, we’ll generously assume that PM never proves any false statements — all of its theorems are sure to be true statements. Now then, armed with our friendly assumption, let’s ask ourselves, “What would follow if KG were provable inside PM?”
A Strange Land where “Because” Coincides with “Although”
Indeed, reader, let’s posit, you and I, that KG is provable in PM, and then see where this assumption — I’ll dub it the “Provable-KG Scenario” — leads us. The ironic thing, please note, is that KG itself doesn’t believe the Provable-KG Scenario. Perversely, KG shouts to the world, “I am not provable!” So if we are right about KG, dear reader, then KG is wrong about itself, no matter how loudly it shouts. After all, no formula can be both provable (as we claim KG is) and also unprovable (as KG claims to be). One of us has to be wrong. (And for any formula, being wrong means being false. The two terms are synonyms.) So… if the Provable-KG Scenario is the case, then KG is wrong (= false).
All right. Our reasoning started with the Provable-KG Scenario and wound up with the conclusion “KG is false”. In other words, if KG is provable, then it is also false. But hold on, now — a provable falsity in PM?! Didn’t we just declare firmly, a few moments ago, that PM never proves falsities? Yes, we did. We agreed with the universal logicians’ belief that PM is consistent. If we stick to our guns, then, the Provable-KG Scenario has to be wrong, because it leads to Russell’s worst nightmare. We have to retract it, cancel it, repudiate it, nullify it, and revoke it, because accepting it led us to a conclusion (“PM is inconsistent”) that we know is wrong.
Ergo, the Provable-KG Scenario is hereby rejected, which leaves us with the opposite scenario: KG is not provable. Now the funny thing is that this is exactly what KG is shouting to the rooftops. We see that what KG proclaims about itself — “I’m unprovable!” — is true. In a nutshell, we have established two facts: (1) KG is unprovable in PM; and (2) KG is true.
We have just uncovered a very strange anomaly inside PM: here is a statement of arithmetic (or number theory, to be slightly more precise) that we are sure is true, and yet we are equally sure it is unprovable — and to cap it off, these two contradictory-
