# Logic for People

Leading programming language theorist Robert Harper refers to so-called constructive or intuitionistic logic as “logic as if people mattered”. There is a fascinating convergence of ideas here. In the early 20th century, Dutch mathematician L. E. J. Brouwer developed a philosophy of mathematics called intuitionism. He emphasized that mathematics is a human activity, and held that every proof step should involve actual evidence discernible to a human. By contrast, mathematical Platonists hold that mathematical objects exist independent of any thought; formalists hold that mathematics is a meaningless game based on following rules; and logicists argue that mathematics is reducible to formal logic.

For Brouwer, a mathematical theorem is true if and only if we have a proof of it that we can exhibit, and each step of that proof can also be exhibited. In the later 19th century, many new results about infinity — and infinities of infinities — had been proved by what came to be called “classical” means, using proof by contradiction and the law of excluded middle. But from the time of Euclid, mathematicians have always regarded reproducible constructions as a better kind of proof. The law of excluded middle is a provable theorem in any finite context. When the law of excluded middle applies, you can conclude that if something is not false it must be true, and vice versa. But it is not possible to construct any infinite object.

The only infinity we actually experience is what Aristotle called “potential” infinity. We can, say, count a star and another and another, and continue as long as you like, but no actually infinite number or magnitude or thing is ever available for inspection. Aristotle famously defended the law of excluded middle, but in practice only applied it to finite cases.

In mathematics there are conjectures that are not known to be true or false. Brouwer would say, they are neither true nor false, until they are proved or disproved in a humanly verifiable way.

The fascinating convergence is that Brouwer’s humanly verifiable proofs turn out also to exactly characterize the part of mathematics that is computable, in the sense in which computer scientists use that term. Notwithstanding lingering 20th century prejudices, intuitionistic math actually turns out to be a perfect fit for computer science. I use this in my day job.

I am especially intrigued by what is called intuitionistic type theory, developed by Swedish mathematician-philosopher Per Martin-Löf. This is offered simultaneously as a foundation for mathematics, a higher-order intuitionistic logic, and a programming language. One might say it is concerned with explaining ultimate bases for abstraction and generalization, without any presuppositions. One of its distinctive features is that it uses no axioms, only inference rules. Truth is something emergent, rather than something presupposed. Type theory has deep connections with category theory, another truly marvelous area of abstract mathematics, concerned with how different kinds of things map to one another.

What especially fascinates me about this work are its implications for what logic actually is. On the one hand, it puts math before mathematical logic– rather than after it, as in the classic early 20th century program of Russell and Whitehead — and on the other, it provides opportunities to reconnect with logic in the different and broader, less formal senses of Aristotle and Kant, as still having something to say to us today.

Homotopy type theory (HoTT) is a leading-edge development that combines intuitionistic type theory with homotopy theory, which explores higher-order paths through topological spaces. Here my ignorance is vast, but it seems tantalizingly close to a grand unification of constructive principles with Cantor’s infinities of infinities. My interest is especially in what it says about the notion of identity, basically vindicating Leibniz’ thesis that what is identical is equivalent to what is practically indistinguishable. This is reflected in mathematician Vladimir Voevodsky’s emblematic axiom of univalence, “equivalence is equivalent to equality”, which legitimizes much actual mathematical practice.

So anyway, Robert Harper is working on a variant of this that actually works computationally, and uses some kind of more specific mapping through n-dimensional cubes to make univalence into a provable theorem. At the cost of some mathematical elegance, this avoids the need for the univalence axiom, saving Martin-Löf’s goal to avoid depending on any axioms. But again — finally getting to the point of this post — in a 2018 lecture, Harper says his current interest is in a type theory that is in the first instance computational rather than formal, and semantic rather than syntactic. Most people treat intuitionistic type theory as a theory that is both formal and syntactic. Harper recommends that we avoid strictly equating constructible types with formal propositions, arguing that types are more primitive than propositions, and semantics is more primitive than syntax.

Harper disavows any deep philosophy, but I find this idea of starting from a type theory and then treating it as first of all informal and semantic rather than formal and syntactic to be highly provocative. In real life, we experience types as accessibly evidenced semantic distinctions before they become posited syntactic ones. Types are first of all implicit specifications of real behavior, in terms of distinctions and entailments between things that are more primitive than identities of things.

# New Approaches to Modality

I periodically peek at the groundbreaking work on formal systems that is going on in homotopy type theory (HoTT), and in doing so just stumbled on an intriguing treatment of modal HoTT that seems much more philosophically promising to me than standard 20th century modal logic.

Types can be taken as formalizing major aspects of the Aristotelian notions of substance and form. Type theory — developed by Swedish philosopher Per Martin-Löf from early 20th century work by the British philosopher Bertrand Russell and the American mathematician Alonzo Church — is the most important thing in the theory of programming languages these days. It is both a higher-order constructive logic and an abstract functional programming language, and was originally developed as a foundation for constructive mathematics. Several variants of type theory have also been used in linguistics to analyze meaning in natural language.

Homotopy type theory combines this with category theory and the categorical logic pioneered by American mathematician William Lawvere, who was also first suggested a category-theory interpretation of Hegelian logic. HoTT interprets types as paths between topological spaces, higher-order paths between paths, and so on, in a hierarchy of levels that also subsumes classical logic and set theory. It is a leading alternative “foundation” or framework for mathematics, in the less epistemologically “foundationalist” spirit of previous proposals for categorical foundations. It is also a useful tool for higher mathematics and physics that includes an ultra-expressive logic, and has a fully computational interpretation.

There is a pretty readable new book on modal HoTT by British philosopher David Corfield, which also gives a nice introductory prose account of HoTT in general and type theory in general. (I confess I prefer pages of mostly prose — of which Corfield has a lot — to forests of symbolic notation.) Corfield offers modal HoTT as a better logic for philosophy and natural language analysis than standard 20th century first-order classical logic, because its greater expressiveness allows for much richer distinctions. He mentions Brandom several times, and says he thinks type theory can formally capture many of Brandom’s concerns, as I previously suggested. Based on admittedly elementary acquaintance with standard modal logic, I’ve had a degree of worry about Brandom’s use of modal constructs, and this may also help with that.

The worry has to do with a concept of necessity that occasionally sounds overly strong to my ear, and is related to my issues with necessity in Kant. I don’t like any universal quantification on untyped variables, let alone applied to all possible worlds, which is the signature move of standard modal logic. But it seems that adding types into the picture changes everything.

Before Corfield brought it to my attention, I was only dimly aware of the existence of modal type theory (nicely summarized in nLab). This apparently associates modality with the monads (little related to Leibnizian ones) that I use to encapsulate so-called effects in functional programming for my day job. Apparently William Lawvere already wrote about geometric modalities, in which the modal operator means something like “it is locally the case that”. This turns modality into a way of formalizing talk about context, which seems far more interesting than super-strong generalization. (See also Modality and Variation; Deontic Modality; Redding on Morals and Modality).

It also turns out Corfield is a principal contributor to the nLab page I previously reported finding, on Hegel’s logic as a modal type theory.

Independent of his discussion of modality, Corfield nicely builds on American programming language theorist Robert Harper’s notion of “computational trinitarianism”, which stresses a three-way isomorphism between constructive logic, programming languages, and mathematical category theory. The thesis is that any sound statement in any one of these fields should have a reasonable interpretation in both of the other two.

In working life, my own practical approach to software engineering puts a high value on a kind of reasoning inspired by a view of fancy type theory and category theory as extensions or enrichments of simple Aristotelian logic, which on its formal side was grounded in the composition of pairs of informally generated judgments of material consequence or material incompatibility. I find the history of these matters fascinating, and view category theory and type theory as a kind of vindication of Aristotle’s emphasis on composition (or what could be viewed as chained function application, or transitivity of implicit implication, since canonical Aristotelian propositions actually codify material inferences) as the single most important kind of formal operation in reasoning.

# Categorical Hegel?

I just discovered a book-length nLab web draft with extremely detailed interpretation of Hegel’s Science of Logic into higher category theory and homotopy type theory. (Reading category theory into Hegel was originally suggested by William Lawvere in the 1960s.) A lot of it is way beyond me, but there is much of interest. nLab in general hosts world-class work in math and logic, as well as applications of it to physics and philosophy. Remarks there about historical philosophers are uneven in quality, but a number of them are interesting, and the more mathematical or logical they are, the better the quality gets. The aforementioned draft does reference old, inadequate generalizations about Hegel as “mystical”, but the detail and scope of the interpretation into state-of-the-art mathematics are awe-inspiring. It also includes a nice formalization of Aristotelian logic, which is mathematically much simpler and relatively easy to understand. I previously found a much shorter page there that explicitly mentions Brandom, and connects with his interest in modal logic. (See also Identity, Isomorphism; Categorical “Evil”; Higher Order.)

# Propositions, Terms

Brandom puts significant emphasis on Kant and Frege’s focus on whole judgments — contrasted with simple first-order terms, corresponding to natural-language words or subsentential phrases — as the appropriate units of logical analysis. The important part of this is that a judgment is the minimal unit that can be given inferential meaning.

All this looks quite different from a higher-order perspective. Mid-20th century logical orthodoxy was severely biased toward first-order logic, due to foundationalist worries about completeness. In a first-order context, logical terms are expected to correspond to subsentential elements that cannot be given inferential meaning by themselves. But in a higher-order context, this is not the case. One of the most important ideas in contemporary computer science is the correspondence between propositions and types. Generalized terms are interpretable as types, and thus also as propositions. This means that (higher-order) terms can represent instances of arbitrarily complex propositions. Higher-order terms can be thus be given inferential meaning, just like sentential variables. This is all in a formal context rather than a natural-language one, but so was Frege’s work; and for what it’s worth, some linguists have also been using typed lambda calculus in the analysis of natural language semantics.

Suitably typed terms compose, just like functions or category-theoretic morphisms and functors. I understand the syllogistic principle on which Aristotle based a kind of simultaneously formal and material term inference (see Aristotelian Propositions) to be just a form of composition of things that can be thought of as functions or typed terms. Proof theory, category theory, and many other technical developments explicitly work with composition as a basic form of abstract inference. Aristotle developed the original compositional logic, and it was not Aristotle but mid-20th century logical orthodoxy that insisted on the centrality of the first-order case. Higher-order, compositionally oriented logics can interpret classic syllogistic inference, first-order logic, and much else, while supporting more inferentially oriented semantics on the formal side, with types potentially taking pieces of developed material-inferential content into the formal context. We can also use natural-language words to refer to higher-order terms and their inferential significance, just as we can capture a whole complex argument in an appropriately framed definition. Accordingly, there should be no stigma associated with reasoning about terms, or even just about words.

In computer-assisted theorem-proving, there is an important distinction between results that can be proved directly by something like algebraic substitution for individual variables, and those that require a more global rewriting of the context in terms of some previously proven equivalence(s). At a high enough level of simultaneous abstraction and detail, such rewriting could perhaps constructively model the revision of commitments and concepts from one well-defined context to another.

The potential issue would be that global rewriting still works in a higher-order context that is expected to itself be statically consistent, whereas revision of commitments and concepts taken simply implies a change of higher-level context. I think this just means a careful distinction of levels would be needed. After all, any new, revised genealogical recollection of our best thoughts will be in principle representable as a new static higher-order structure, and that structure will include something that can be read as an explanation of the transition. It may itself be subject to future revision, but in the static context that does not matter.

The limitation of such an approach is that it requires all the details of the transition to be set up statically, which can be a lot of work, and it would also be far more brittle than Brandom’s informal material inference. (See also Categorical “Evil”; Definition.)

I am fascinated by the fact that typed terms can begin to capture material as well as purely formal significance. How complete or adequate this is would depend on the implementation.