Here is my pet peeve: theoretical computer scientists misuse the word “provably”. Stop it. Stop it!

Theoretical computer science is closer to mathematics than it is to computer science. There are definitions, theorems and proofs. Theoretical computer scientists must understand mathematical terminology. The words “proof” and “provable” are in the domain of mathematical logic. A statement is provable if it has a proof in a given formal system. It makes no sense to say “provable” without specifying or implying a specific proof system.

But theoretical computer scientists say things like (I just googled these randomly) “A Provably Optimal Algorithm for Crowdsourcing” and “A Provably Correct Learning Algorithm for Latent-Variable PCFGs” and even “provably true”.

So what is a “provably optimal algorithm” ? It is an algorithm for which there exists a proof of optimality. But why would you ever want to say that in the title of your paper? I can think of several reasons:

- You proved that there exists a proof of optimality but did not actually find the proof itself. This of course is highly unlikely, but that does even not matter, for as soon as we know there exists a proof, the algorithm
**is**optimal. Just say “optimal algorithm” and the world will be a more exact place. - Your paper is an intricate piece of logic which analyzes existence of proofs of optimality in some super-important formal systems. This of course is not what theoretical computer scientists do for the most part. Any paper which actually did this, would instead say something like “$\mathrm{Ind}(\Sigma^0_1)$-provability of optimality”.
- You distinguish between algorithms which are optimal and those which are optimal and you proved they’re optimal. In that case we should turn tables: if you ever publish a claim of optimality without having proved it, put that in your title and call it “Unproved optimal algorithm”.
- You proved that your algorithm is optimal, showed the proof to the anonymous referee and the editor, and then published your optimal algorithm without proof. You want to tease your readers by telling them “there is a proof but it’s a secret”. If this is what you meant to convey, then the title was well chosen.

To see that “provable” is just a buzzword, consider what it would mean to have the opposite, that is an “unprovably optimal algorithm”. That is an algorithm which is optimal, but there exists no proof of optimality. Such a thing can be manufactured by a logician, probably, but is certainly not found in everyday life.

As someone is going to say that “provably true” makes sense, let me also comment on that. A statement is true if it is valid in all models. So “provably true” would mean something like “there exists a proof in a given formal system that the statement is valid in all models”. Well, I will not deny that a situation might arise in which this sort of thing is precisely what you would consider, but I will also bet you that it is far more likely that “provably true” should just be replaced by either “provable” or “true”, depending on the particularities of the situation.

**As a rule of thumb, unless you are a logician, if you feel like using the word “provably”, just skip it.**

And as long as I am ranting, please stop introducing every single concept with “informally” and prefixing every single displayed formula with “formally”. Which is it,

- you think that “informal” descriptions are somehow unworthy or broken, and you should therefore alert the reader that you’re lying to them, or
- you think that “formal” descriptions are an unnecessary burden which must be included in the paper to satisfy the gods of mathematics?

If the former, stop lying to your readers, and if the latter stop doing theoretical computer science.

Now I will go on to refereeing that pile of POPL papers which must contain at least a dozen misuses of “provably” and two dozen false formal/informal dichotomies.

]]>Let $\Omega$ be the set of truth values (in Coq this would be the setoid whose underlying type is $\mathsf{Prop}$ and equality is equivalence $\leftrightarrow$, while in HoTT it is the h-propostions). Call a truth value $p : \Omega$ **intermediate** if it is neither true nor false, i.e., $p \neq \bot$ and $p \neq \top$. Such a “third” truth value $p$ is proscribed by excluded middle.

The puzzle is to explain how the following two facts fit together:

**“There is no intermediate truth value”**is an intuitionistic theorem.**There are models of intuitionistic logic with many truth values.**

Mind you, excluded middle says “every truth value is either $\bot$ or $\top$” while we are saying that there is no truth value different from $\bot$ and $\top$:

$$\lnot \exists p : \Omega \,.\, (p \neq \top) \land (p \neq \bot).$$

Coq proves this:

Theorem no_intermediate: ~ exists p : Prop, ~ (p <-> True) /\ ~ (p <-> False). Proof. firstorder. Qed.

Note that we replaced $=$ with $\leftrightarrow$ because equality on $\Omega$ is equivalence on $\mathsf{Prop}$ in Coq (this would not be neccessary if we used HoTT where h-propositions enjoy an extensionality principle). You should also try proving the theorem by yourself, it is easy.

A model of intuitionistic mathematics with many truth values is a sheaf topos $\mathsf{Sh}(X)$ over a topological space $X$, so long as $X$ has more than two open sets. The global points of the sheaf of truth values $\Omega$ are the open subsets of $X$, and more generally the elements of $\Omega(U)$ are the open subsets of $U$.

So, if we take an intermediate open set $\emptyset \subset U \subset X$, should it not be an intermediate truth value? Before reading on you should stop and think for yourself.

Really, stop reading.

Let us calculate which open set (truth value) is

$$(U \neq \top) \land (U \neq \bot).$$

Because $U = \top$ is equivalent to $U$ and $U = \bot$ to $\lnot U$ our formula is equivalent to

$$\lnot U \land \lnot\lnot U.$$

Remembering that negation is topological exterior we get

$$\mathsf{Ext}(U) \cap \mathsf{Ext}(\mathsf{Ext}(U))$$

which is empty,

$$\emptyset.$$

Indeed, $U$ is *not* a counterexample!

We have here a typical distinction between *internal* and *external* language:

- The mathematicians inside the topos think and speak
*locally*. They ask not “Is this statement true?” but “*Where*is this statement true?” If you aks them a yes/no question they will answer by giving you an open subset of $X$. They will conclude that $U \neq \top$ holds on the exterior of $U$, and $U \neq \bot$ on the double exterior of $U$, and that nowhere are they true both together. - The mathematicians outside the topos (that’s us) understand $(U \neq \top) \land (U \neq \bot)$ differently: it is about comparing the open set $U$ to the open sets $X$ and $\emptyset$ as elements of the topology of $X$. For them “yes” and “no” are valid answers, and no other.

By the way, the mathematicians on the inside also think that “yes” and “no” are valid answers, and there is no other – that is precisely the statement “there is no intermediate truth value” – but they think of it as “holding everywhere on $X$”.

There are of course many situations where the difference between iternal and external language may create confusion. For example, if $X = \mathbb{N}$ is a countable discrete space then the object of natural numbers is the sheaf of *all* maps into $\mathbb{N}$, of which there are uncountably many. Thus on the outside we “see” that there are uncountably many natural numbers in $\mathsf{Sh}(X)$. Those on the inside would of course disagree. This is an amusing situation, sort of a reverse of Skolem’s paradox.

**Download slides:** The troublesome reflection rule (TYPES 2015) [PDF].

A position is available for a PhD student at the University of Ljubljana in the general research area of modelling and reasoning about computational effects. The precise topic is somewhat flexible, and will be decided in discussion with the student. The PhD will be supervised by Alex Simpson who is Professor of Computer Science at the Faculty of Mathematics and Physics.

The position will be funded by the Effmath project (see project description). Full tuition & stipend will be provided.

The candidate should have a master’s (or equivalent) degree in either mathematics or computer science, with background knowledge relevant to the project area. The student will officially enrol in October 2015 at the University of Ljubljana. No knowledge of the Slovene language is required.

The candidates should contact Alex.Simpson@fmf.uni-lj.si by email as soon as possible. Please include a short CV and a statement of interest.

]]>The candidate should have as many of the following desiderata as possible, and at the very least a master’s degree (or an equivalent one):

- a master’s degree in mathematics, with good knowledge of computer science
- a master’s degree in computer science, with good knowledge of mathematics
- experience with functional programming
- experience with proof assistants
- familiarity with homotopy type theory

The student will officially enrol in October 2015 at the University of Ljubljana. No knowledge of Slovene is required. However, it is possible, and even desirable, to start with the actual work (and stipend) earlier, as soon as in the spring of 2015. The candidates should contact me by email as soon as possible. Please include a short CV and a statement of interest.

**Update 2015-03-28:** the position has been taken.