Nerd alert!! I’m a little scared to write this because I don’t fully grasp it yet and I’ll probably explain things wrong, but how else do you learn? So here we go. This review has two sections: First, an overview of what this book is about (mostly for myself, to see if I can explain it, and for my lovely followers who tend not to be the mathematics types). And second, an overview of what I thought of the book itself, which will appeal more to people who have read or are considering reading this book. Here’s to deepening our wonder!
What On Earth Is This About?
I first heard about Gödel’s incompleteness theorem through a Veritasium YouTube video whose description begins: “There is a hole at the bottom of math, a hole that means we will never know anything with certainty.” I have claimed as my life motto a Venn diagram I saw once with one circle holding “science” and one “art” and in the center “wonder.” I am fascinated by the intersection of different disciplines and how they move us toward wonder. And this theorem, which I'll explain more in a minute, is the glorious epitome of that: math and philosophy harmonizing and creating an opera that has shaken both fields with its majesty and unexpectedness.
Though the details of the proof are difficult, the overall strategy is—happily!—almost simplicity itself. Simple but strange, as one would expect of a proof that draws so close to the edge of self-contradiction, proving that there are true arithmetical propositions that are not provable.
So, Gödel’s incompleteness theorem: A basic summary, in Goldstein’s words, is this: “A system rich enough to contain arithmetic cannot be both consistent and complete.” This is referring to mathematical systems (like your basic Euclidean geometry which is built upon rules, or axioms, like: parallel rules never touch). Consistency means there are no contradictions (so if Euclidean geometry is consistent, it is not possible for parallel lines both to touch and never touch), and completeness means you can prove this consistency in the system (so given all the basic axioms in Euclidean geometry, like parallel lines cannot touch, can you prove that all those principles must be true?).
Here's why people were arguing about this in the first place. In the early 1900s philosophers known as logical positivists were trying to eliminate any pesky paradoxes and ambiguities in thinking and language. The best way to do this, they thought, was to get rid of metaphysics altogether since it can’t be proven empirically (i.e., through our senses). Parallel to them in mathematics, a group known as the formalists had a similar goal of eliminating paradoxes and all such messiness from math. They wanted to do this by reducing all arithmetic to symbol games. Math is not about external realities, like numbers, that exist, they argued. It’s just a manipulation of the rules we set. Two plus two doesn’t equal four because “two” really exists out there and we have found a way to describe it and how it works. Two plus two equals four because we have determined that’s what two and four mean and how they’re going to work in this system.
Gödel, however, was neither a logical positivist or a formalist. He was a Platonist. He believed that there were actual external realities to which our language and our logic conform. We do not invent systems, we discover them.
If you treat numbers like this, however, you can run into some messy problems. Consider the famous self-referential paradox of set theory: If a set contains all sets that do not contain themselves, it cannot contain itself, otherwise it should be in the set. But if it does not contain itself, then it is a set that does not contain itself and therefore should be in the set.
The formalists hated these conundrums and wanted a formal proof that would get rid of paradoxes like those in set theory. Here's where completeness comes in. If this proof could be “drained of the descriptive content” (i.e., take away the idea that symbols for numbers actually stand for numbers that exist and have real properties whether we have a symbol for them or not) and if, using only the rules of logic and arithmetic, this proof could show that a mathematical system was both consistent and complete, then we wouldn't have to deal with any of the paradoxes that come when you treat numbers like independently existing entities.
Then along came Gödel, motivated by the belief that you could not treat numbers the way the formalists wanted to, that numbers did exist themselves. His famous theorem proved that this proof the formalists desired is unprovable. Specifically, it proved that there are true statements that are unprovable in a consistent system, which means it’s incomplete—the system can’t tell you everything you need to know about itself. So if you want consistency, you can't have at the same time completeness. As Goldstein says,
The second incompleteness theorem put formalism in an impossible bind: the formalist incentive was to banish the opacity of the nature of the thing in itself (space, numbers, sets) for the transparency of formal systems. But it's of the highest priority that a formal system—drained of the descriptive content that would, so long as the axioms were truly descriptive, ensure its consistency—be proved consistent. This can only be done by going outside the formal system and making an appeal to intuitions that can't themselves be formalized.
And how did Gödel prove this? This is the most exciting part. I can't begin to get into the details—not even Goldstein does—but the basic idea is this. Gödel created Gödel numbering, where logical propositions and numbers have their own special number. Then when you start manipulating those numbers with mathematics, you not only get actual mathematical statements ( like 2 + 2 = 4 ) but those mathematical statements simultaneously map onto what Goldstein calls “metamathematical statements” or philosophical statements about the nature of math.
A very simplistic and kind of inaccurate but hopefully helpful example: If the Gödel number of 2 means, say, “this number is provable,” then in Gödel numbering “2 + 2 = 4” would saying something about the mathematical, numerical nature of 2 (namely, that added to itself you get four) but it would also be saying something about numbers being provable (in addition to whatever philosophical statement +, =, and 4 map onto).
Goldstein uses different metaphors to help explain the genius of this method: Gödel numbering, she says, “allows some propositions to engage in an interesting sort of double-speak, saying something arithmetical and also commenting on their own situation within the formal system, saying whether they're provable;” and it is “basically the idea of encoding, which allows you to move back and forth between the original propositions and the code.”
Think Shakespeare's A Midsummer Night's Dream, a play within a play, how the lines the characters are speaking in the internal play also have meaning about the larger play as a whole. Only now it’s mathematical statements—equations, numbers—that are lines in the internal play, with double meanings that say something about the play as a whole—in this case, the nature of mathematics.
And by the nature of mathematics, I mean those questions the logical positivists wrestled with: Can we know things that are not empirically provable? Is math about actual numbers that we do not control or is it setting up the parameters of a game and making sure we follow them, using symbols we all agree on? Somewhat insanely, Gödel’s theorem answers those questions about mathematics through mathematics.
I found Goldstein’s description about how the proof leads us to these big questions very helpful:
The most straightforward way of understanding intuitions is that they are given to us by the nature of things; again intuition is seen as the a priori analogue to sense perception, a direct form of apprehension. So Gödel’s conclusion, in having something to say about the feasibility (or lack thereof) of eliminating all appeals to intuitions from mathematics might also have a thing or two to say about the actual existence of mathematical objects, like numbers and sets. In other words, the adequacy of formal systems—their consistency and completeness—is linked with the question of the ultimate eliminability of intuitions, which is linked with the question of the ultimate eliminability of mathematical reality, which is the defining question of mathematical realism, or Platonism. It is because of these linkages that Gödel’s conclusions about the limits of formal systems have so much to say.
This brings up what for me was the most interesting part of the book: the idea of mathematical intuition. Goldstein sets this up best:
Sense perception allows us to make contact with what's out there in physical reality. What is the bedrock of mathematical knowledge? Is there something like sense perception in mathematics? Do mathematical intuitions constitute this bedrock?...Mathematical proofs must start from somewhere. Often proofs start with conclusions from other proofs and then the deduce further conclusions from these. But not everything can be proved, otherwise how can we get off the ground? There must be, in mathematics just as in empirical knowledge, the “given.” Given to us through what means? Mathematical intuition is often thought of as the a priori analogue to sensory perception.
An example of these intuitions could be, for example, the idea that parallel lines will never touch. The problem with these intuitions is that they can lead to paradoxes, like that set of all sets that do not contain themselves. Hence why Hilbert and the formalists wanted to be rid of them and have just the pure “game” of math, only holding onto whatever is provable within the rules of the system. As Gödel showed, however, that's not very much. In fact, that doesn't include the foundations upon which the rest of the mathematical system is built.
So if there are true statements that are unprovable, these unprovable true statements must come to us through intuition. And how does intuition operate except by “bumping into” an external reality? And so Gödel’s proof gets us into the deepest of philosophical territory: What is real? How do we know it? Do our senses encompass all there is? What about our reasoning?
And gosh, if the fact that a mathematical proof not only raises those questions but has something to say about them doesn't excite you, then maybe you need a fresh dose of wonder.
The Book:
I really enjoyed the style of this book. It's part biography, part basic mathematical, logical, and philosophical explanation. There's enough biography to understand why Gödel was so interested in the question of completeness and consistency and how he had the unique temperament to pursue it in the mind-bending way he did. There is also very helpful context for the philosophical and mathematic landscape of the time, setting up why this question was so pressing and what other leading intellectual figures (namely the Vienna Circle, Wittgenstein, and David Hilbert) had to say about it.
Intellectual discoveries, they're all epic adventure stories. So you have to have a firm grasp of the world-building, know who your nemeses are, get an inside look at the foibles and geniuses of your hero, and, most of all, understand the magic system which is, in this case, a mishmash of philosophy (specifically, Platonism and positivism) and mathematics (specifically, a bit of set theory and logic).
Goldstein is a wonderful narrator on this breath-taking journey. My one critique is that she veers a bit too close to hagiography at times. She does a good job arguing for why Gödel was such a genius, and since she was a student at Princeton when he was there, I understand her personal connection to him. But she could have toned down the superlatives and instead provided a longer list of examples of the other discoveries and inventions that his theorem has birthed.
I'm a theology major with a pretty solid undergrad foundation in philosophy and an amateur’s interest in mathematics (the highest I got was AP Calculus in high school). It required a close read and intense focus, and it helped that I've watched some YouTube videos about this theorem before. There were definitely some technical components that went over my head but overall this read was not only accessible but exciting.
At one point Goldstein describes Gödel’s theorem as having “heart-stopping beauty,” and I felt that. Her description of it enthralled me—you can tell she's a novelist but she also has the philosophical chops to pull this off. It's a marvelous combination and quite appropriate for the subject matter, a theorem that unites two different fields.
I'm a Christian, and this book led me to worship. Part of what excites me about this theorem is that it points to external, objective reality. (Or at least, it can be interpreted to point to that—a big part of Goldstein's argument is that Gödel’s peers, critics, and followers have consistently misinterpreted him here.) Mathematics, so Gödel’s theorem seems to say, is not simply the “semantic-free mechanical processes of mindless symbol-manipulation.” It is bumping up against something real, even though we cannot prove it in a formal system.
Of course you can't deduce from that the existence (or non-existence) of God. That's part of the whole point of what's so revolutionary about Gödel’s theorem: “Whereof we cannot formalize, thereof we still can know” or, a la early Wittgenstein, “We cannot speak the unspeakable truths, but they exist.” The most important things in life, questions about God and morality and the meaning of life, cannot be spoken of in transparent linguistic systems, packaged in logical proofs, or formalized in equations beyond doubt. But just as we somehow know that two plus two equals four and can build civilizations around that fact and the others that follow from it, we can have high confidence in beliefs about God, ethics, and human nature and build lives upon them.
I do not want to collapse the distinctions between mathematics/logic/science and philosophy/theology/ethics. But I am saying that this discovery of Gödel’s—that there are things that are true but unprovable, that the limits of our reasoning do not necessitate disbelief in what is beyond reason—bolsters my faith in God. It is another example that to believe in the existence of a Master Creator and Sustainer like the Judeo-Christian God—a God who exists outside of us, a reality independent of us that we do not create but discover—is not unreasonable. Of course, for some people Gödel’s proof has persuaded them of the opposite. Fair enough. Such is the problem with messy, unprovable metastatements. This is exactly the kind of thing that Wittgenstein and Hilbert, in their own ways, tried to avoid. And it is exactly the kind of thing that Gödel showed us we cannot escape.