After a productive year at the Institute for Advanced Study, the Univalent Foundations Program has written a book on Homotopy Type Theory (HoTT). The foreword gives a succinct description of the purpose of this book:

We did not set out to write a book. The present work has its origins in our collective attempts to develop a new style of "informal type theory" that can be read and understood by a human being, as a complement to a formal proof that can be checked by a machine. Univalent foundations is closely tied to the idea of a foundation of mathematics that can be implemented in a computer proof assistant. Although such a formalization is not part of this book, much of the material presented here was actually done first in the fully formalized setting inside a proof assistant, and only later "unformalized" to arrive at the presentation you find before you — a remarkable inversion of the usual state of affairs in formalized mathematics.

The danger in writing such a book is to fall into the minefields of logic wars. The authors successfully avoided much of these traps, so logicians from other perspectives can read the book without too much cringing. To avoid unnecessary confusion, I recommend mentally substituting most instances of "set theory" by the more apropos "classical mathematics." Readers from strongly opposing points of view should be prepared for a certain amount of propaganda, which is to be expected in a book written to promote one point of view. Barring these caveats, you will find an enjoyable and well-written book on a very interesting subject. Readers should not be too concerned with the word "homotopy" in the title, homotopy theory is not required background for the book though some basic knowledge of the ideas of topology and homotopy theory helps to understand the motivation behind certain concepts.

Having addressed all the necessary caveats, let’s talk about why this book is interesting and why you should read it…

### What is so hot about HoTT?

The most interesting aspect from my point of view is that HoTT fully supports proof-relevant mathematics, a way of doing mathematics where proofs are real objects that are manipulated on the same level as numbers, sets, functions and all the usual objects of classical mathematics. This is not a brand new idea, logicians have been playing with proofs in this way for a long while, but HoTT brings this idea into the realm of everyday mathematics and that is a major step forward in mathematics.

The key difference with first-order logic is that equality is not primitive. To define a type $A$ one must also define what equality means for $A$. Formally if $x,y:A$ then $x =_A y$ (or $\mathsf{Id}_A(x,y)$) is another type, an identity type, which can be thought of as made of reasons to identify $x$ and $y$. Elements $p:x =_A y$ are often called "paths" by analogy with topolgy. Indeed, these paths can be inverted and concatenated to realize symmetry and transitivity of equality, respectively; reflexivity is realized by a path $\mathsf{refl}_x:x =_A x$. Thus each type $A$ is actually a groupoid rather than a plain set. In fact, since each $x =_A y$ is itself a type with its own identity types and so forth, the type $A$ is actually a kind of $\infty$-groupoid.

It is this rich structure associated with each type is what permits HoTT to support proof relevant mathematics. To get a basic feel of how this works, the statement "$x =_A y$ and $y =_A z$" is interpreted via the product type $(x =_A y)\times(y =_A z)$, whose elements are pairs of paths that explain why $x$ is to be identified with $y$ and why $y$ is to be identified with $z$. Similarly, "$x =_A y$ or $y =_A z$" is interpreted via the coproduct type $(x =_A y) + (y =_A z)$, whose elements are either paths that explain why $x$ is to be identified with $y$ or paths that explain why $y$ is to be identified with $z$. The catch, as you may have guessed from the last example, is that this form of constructive reasoning is intuitionistic and thus not as familiar to mathematicians.

Interestingly, the learning curve for constructive reasoning appears to be much less steep with HoTT than with other constructive frameworks. One of the reasons is that the topological interpretation of the key concepts is very intuitive but more significantly HoTT provides many tools to revert to more familiar territory. The analogue of a plain set in HoTT is a $0$-type: a type $A$ where the identity types $x =_A y$ always contain at most one path. In other words, these are types where the groupoid structure is trivial and contains no other information than how to handle equality of elements. It is consistent with HoTT that the $0$-types form a model of ETCS, a classical theory of sets and functions. Thus, by "truncating" thoughts to $0$-types, one can revert to a more familiar classical setting.

### What is the big deal with univalence?

It is natural to identify things that are not significantly different. For example, the axiom of extensionality in set theory identifies sets that have the same elements since the elements of a set are all that matter in this context. Extensionality for functions identifies functions that agree on all inputs. Univalence is an indiscernibility axiom in the same spirit: it identifies types that are not significantly different.

To make sense of equality for types, we first need to put them in an ambient type, a universe, with its associated identity types. We can’t have a truly universal type since that directly leads to the usual paradoxes of self-reference. Instead, we have a bunch of universes such that each type belongs to some universe and each universe is closed under the basic type formation rules. Once we have a universe $\mathcal{U}$ we can talk about equality of types in $\mathcal{U}$, and because $\mathcal{U}$ is a type we have a lot of freedom in defining what equality means for types in $\mathcal{U}$.

This is exactly what the univalence axiom does. It can be stated elegantly: $(A \simeq B) \simeq (A =_\mathcal{U} B).$ The equivalence relation ${\simeq}$ is similar to isomorphism but it is slightly more permissive. To say $A \simeq B$ requires the existence of a function $f:A \to B$ together with $\ell,r:B \to A$ such that $\ell \circ f$ is homotopic to $\mathsf{id}_A$ and $f \circ r$ is homotopic to $\mathsf{id}_B$. Given two functions $f$ and $g$ of the same dependent product type $\prod_{x:A} B(x)$, a homotopy from $f$ to $g$ is an element of $\prod_{x:A} (f(x) =_{B(x)} g(x))$. So $f$ and $g$ are homotopic if they agree on all inputs, which does not mean that $f = g$ in the absence of function extensionality.

In general type theory, $A \simeq B$ is definitely a good way to say "$A$ and $B$ are not siginificantly different" and thus univalence arguably captures the right indiscernibility axiom for type theory. The surprising fact is that such a strong axiom does not appear to collapse more than it should. The benefits of univalence are interesting and need to be explored further.

I still need to digest the book so this is probably only the first of many posts on HoTT. The next posts will undoubtedly wander deeper into the technical levels of HoTT. There are a few interesting leads but nothing definite yet.

There is only one thing that bugs me right now, which is the way universes are handled in the book. However, since these assumptions do not appear to be crucial for the development of HoTT and there are plenty of alternatives out there, I’m not overly concerned about this at the moment.

I will eventually need to talk about higher inductive types. These are really interesting and I’m happy to see that the book devotes an entire chapter to them. This is a very interesting outgrowth of this project and which deserves study even independently of HoTT.

In [2], Joel David Hamkins proposed a set of axioms for the set-theoretic multiverse. Several of these axioms reflect the typical world many set theorists live in, namely that generic extensions, inner models and the like are all legitimate universes. Some set theorists prefer the universe perspective where one such universe is singled out as more legitimate than others and other set theorists prefer to think that any universe is as legitimate as any other. Hamkins is of the latter view and, in many ways, his view is even radically opposed to the universe perspective. Indeed, some of Hamkins’s axioms take the form of “mirages” that gradually eliminate the possibility of singling out a preferred universe. These can be formulated as follows.1

Uncountability Mirage
Every universe is countable from the perspective of a larger universe.
Undefinability Mirage
Every universe is constructible from the perspective of a larger universe.
Wellfoundedness Mirage
Every universe is not wellfounded from the perspective of a larger universe.
Finiteness Mirage
Every universe has a nonstandard $\omega$ from the perspective of a larger universe.

The Undefinability Mirage is a fascinating axiom that I hope to talk about in the near future but it does not directly contradict the universe perspective. The Uncountability Mirage is startling but it isn’t earth-shattering on its own. However, the last two mirages are fundamentally incompatible with the universe perspective. The Finiteness Mirage — which actually implies the Wellfoundedness Mirage — is especially striking since it asserts that no universe understands true finiteness. This is a chilling thought for most mathematicians, even those who do not espouse the classical universe view. After reading Hamkins’s account, I wondered what kind of arithmetical consequences the multiverse axioms have. This is an important question since an arithmetical disagreement between the multiverse view and the universe view would make the divide much more than a different perspective.

Set Theory has a fantastic and legendary history. At the end, it left us with ZFC, which is currently recognized as the foundation of mathematics. This state of affairs is arguably one of the best possible outcomes for the foundational crisis that plagued mathematics in the early 20th century. However, the choice to adopt ZFC was by no means unanimous, dissent has always been present, often with good reason. Whether propelled by inertia or by sheer power, for better or for worse, ZFC is now approaching 100 years of reign as the foundation of mathematics.

In the 1970s, Radu Diaconescu [1] showed that the Axiom of Choice implies the Principle of Excluded Middle. More specifically, he showed that every topos that satisfies (a very mild form of) the Axiom of Choice is Boolean. Diaconescu’s argument applies in many other contexts too. In this post, I will present a variant of that argument. I will keep the context of the argument deliberately ambiguous to demonstrate its broad applicability.

I’ve been thinking a bit more about the issues brought up in Paris–Harrington and Typing and Santa Exists!

These issues seem related to Borges’s ornithological argument for the existence of God:1

Cierro los ojos y veo una bandada de pájaros. La visión dura un segundo o acaso menos; no sé cuántos pájaros vi. ¿Era definido o indefinido su número? El problema involucra el de la existencia de Dios. Si Dios existe, el número es definido, porque Dios sabe cuántos pájaros vi. Si Dios no existe, el número es indefinido, porque nadie pudo llevar la cuenta. En tal caso, vi menos de diez pájaros (digamos) y más de uno, pero no vi nueve, ocho, siete, seis, cinco, cuatro, tres o dos. Vi un número entre diez y uno, que no es nueve, ocho, siete, seis, cinco, etcétera. Ese número entero es inconcebible; ergo, Dios existe.2

Although not intended to be taken seriously, Borges’s argument rests on the decidability of equality for natural numbers. Here is a more mathematical example pointed out by Noam Elkies.3 Consider $\R$ as a vector space over $\Q$ and let $V$ be the subspace spanned by $\set{1,e,\pi}$. We know that $\dim(V) = 2 \lor \dim(V) = 3$, but which one is it?

The Santa post raises the issue whether set membership should be decidable, which is even less clear than the issue with equality of natural numbers. However, the theorem there is easily modified to prove the existence of God in naïve set theory without assuming the decidability of set membership.

The Paris–Harrington post raises a more subtle issue. It is pointed out there that the notion of relative largeness ($\min(H) \leq |H|$) used in the Paris–Harrington theorem is ill-typed in that it requires the comparison of two different types of natural numbers: the minimal element of a set of natural numbers and the cardinality of that set. In that case, type-theoretic reasons alone lead to the questionability of this comparison.

Anyway, I don’t have any new insights on decidability issues. I just wanted to point out the relationship between these posts, and I couldn’t resist creating a tag for Jorge Luis Borges…

#### Notes

1. I would like to thank Mariano Suárez-Alvarez and José Figueroa-O’Farrill for reminding me of this argument in a meta discussion on MathOverflow regarding question 77068.
2. Translation by Mildred Boyer: I close my eyes and see a flock of birds. The vision lasts a second or perhaps less; I don’t know how many birds I saw. Were they a definite or an indefinite number? This problem involves the question of the existence of God. If God exists, the number is definite, because how many birds I saw is known to God. If God does not exist, the number is indefinite, because nobody was able to take count. In this case, I saw fewer than ten birds (let’s say) and more than one; but I did not see nine, eight, seven, six, five, four, three, or two birds. I saw a number between ten and one, but not nine, eight, seven, six, five, etc. That number, as a whole number, is inconceivable; ergo, God exists.
3. See the comment thread to the MathOverflow question 77068.

Here is a theorem that I like to mention during the first week when I teach set theory.

Theorem. Santa exists!