On a relevance criterion

Logicians study logics – plural. There are different logics for different reasoning tasks. Classical logic, the flavour taught to undergraduate students of all persuasions, falls apart when confronted with the kinds of reasoning that people do effortlessly every day. My favourite way to break classical logic involves an innocent “if” and “or”.

Ponder the following sentence (based on an example by Alf Ross, 1944):

If Alex posted the letter [P], then Alex posted the letter [P] or Alex set fire to the letter [F].

If you think this sentence is true, then your interpretation and reasoning are compatible with translating it into classical logic using the material conditional (\(\Rightarrow\)) for the “if” and inclusive disjunction (\(\lor\)) for the “or”. You could write it like this and it’s trivially true: \(P \Rightarrow (P \lor F)\).

Some people are perfectly content with this interpretation, but many think the sentence is fishy and false.  There are a number of ways to explain what has happened.

One is to assume that the issue is language pragmatics rather than logic. Pragmatics studies the ways in which context and social conventions for communication affect people’s interpretation of language. According to one theory of communication (see Liza Verhoeven’s 2007 explanation), asserting that you posted the letter or burned it under the assumption that you posted it violates principles of cooperativeness. These principles affect the meaning of a sentence and its truth, so in this case the sentence is false.

Another way to make sense of what has gone wrong is using a relevance criterion devised by Gerhard Schurz (1991). The first step we need to take is to transform the “if” into an argument with a single premise and conclusion.

Premise: Alex posted the letter [P].
Conclusion: Alex posted the letter [P] or Alex set fire to the letter [F].

This is an uncontroversial step in classical logic, e.g., application of a rule for introducing an “if” in natural deduction.

Schurz introduces a criterion for a conclusion relevance that roughly goes as follows. The starting point is an argument that is valid according to classical logic. That’s the case for the argument above. If there are any terms in the conclusion that can be substituted with arbitrary alternatives without affecting the argument’s validity, then the conclusion is irrelevant. Otherwise the conclusion is relevant.

For our letter example, we can replace “Alex set fire to the letter” with anything and it has no effect on the validity of the argument. Alex opened the letter. Alex scribbled on the letter. Alex swallowed the letter. The letter was a surrealist painting. The letter was the size of house. And so on. No substitution in the second half of the conclusion can affect the validity of the argument, so the conclusion is irrelevant.

How about an argument where the conclusion is relevant? The trick is to ensure that everything in the conclusion is… relevant. That’s what I like about the criterion: it formalises (and the details are fiddly) an intuitive property of arguments. Here’s an easy example:

Premise: It’s raining and I left my umbrella at home
Conclusion: I left my umbrella at home and it’s raining

This is an example of the conjunction, “and”, being commutative in classical logic: the order of the conjuncts in the sentence (the parts on either side of “and”) doesn’t affect its truth. There are many ways to edit the conclusion so that the argument is no longer valid. For instance replace one or both of the conjuncts with “I posted a letter”. Then the conclusion doesn’t follow from the premise since the premise doesn’t tell us anything about a letter.

Colleagues and I explored people’s interpretations of these kinds of sentence about a decade ago in the context of an alleged paradigm shift in the psychology of reasoning. Read all about it. I was reminded of this again as Google Scholar dutifully notified me that Michał Sikorski recently cited it (thank you kindy Michał!).

Drawing an is-ought

Hume’s (1739) Treatise famously argued that we cannot infer an “ought” from an “is”. This has presented an enduring problem for science: how should we produce a set of recommendations for what should be done following the results of a study? If a new cancer treatment dramatically improves remission rates, should study authors simply shrug, present the results, and leave the recommendations to politicians? What if a treatment causes significant harms – can we recommend that the treatment be banned? Or suppose we have ideas for future studies that should be carried out and want to summarise them in the conclusions…? Even doing this would be ruled out by Hume.

The solution, if it is one, is that any recommendations require a set of premises stating our values. These values necessarily assert something beyond the evidence, for instance that if a treatment is effective then it should be provided by the health service. In practice, such values are often left implicit and assumed to be shared with readers. But there are interesting examples where it is apparently possible to draw an is-ought inference without assuming values.

One example, due to Mavrodes (1964), begins with the premise

If we ought to do A, then it is possible to do A.

This seems reasonable enough. It would, for instance, be horribly dystopian to require that people behave a particular way if it were impossible for them to do so. Games like chess and tennis have rules that are possible – if they were impossible then it would make playing the games challenging. Let’s see what happens if we apply a little logic to this premise.

Sentences of the form

If A, then B

are equivalent to those of the contrapositive form

If not-B, then not-A

This can be seen in the truth table below, where 1 denotes true and 0 denotes false. The values of the last two columns are equivalent:

A B not-A not-B If A, then B If not-B, then not-A
1 1 0 0 1 1
1 0 0 1 0 0
0 1 1 0 1 1
0 0 1 1 1 1

Together, this means that if we accept the premise

If we ought to do A, then it is possible to do A,

and the rules of classical logic, we must also accept

If it is not possible to do A, then it is not the case that we ought to do A.

But here we have an antecedent that is an “is” and a consequent that is an “ought”: logic has licenced an is-ought!

Worry not: there has been debate in the literature… See Gillian Russell (2021) for a recent analysis.


Mavrodes, G. I. (1964). “Is” and “Ought.” Analysis, 25(2), 42–44.

Russell, G. (2021). How to Prove Hume’s Law. Journal of Philosophical Logic. In press.

A psychoanalyst walks into a bar(red subject)

A psychoanalyst walks into a bar with a book on logic and set theory. He orders a whisky. And another. Twelve hours and a lock-in later, all he has to show for the evening is a throbbing headache and some indecipherable bollocks scrawled on a napkin.

That’s the only conceivable explanation for these diagrams from The Subversion of the Subject and the Dialectic of Desire in the Freudian Unconscious, by Jacques Lacan (published in the Écrits collection):

But, surely this notation means something? After all, Lacan is famous and academics across the world sweat whisky to try to understand his genius.

Also the notion  f(x) is a function, f, applied to argument x — that’s recognisable from maths. So the I(A) and s(A) must mean something…?

Here is a brief interlude on functions to show how they can be introduced and used. The Fibonacci sequence, which pops up in all kinds of interesting places in nature, is defined as follows:

f(0) = 0,
f(1) = 1,
f(n) = f(n-1) + f(n-2), for n > 1.

In English, this says that the first two numbers in the sequence are 0 and 1. The numbers following are obtained by summing the previous two: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, …

If you tell it a number (e.g., 0, 1, 2, …) then it replies with the respective number in the sequence (first, second, third, …). It might look a bit scary if you haven’t seen the notation before, but have a look at these examples showing how the sums are done. You start with 0 and 1 and then to get the numbers for larger values, check back at your previous scribbles and fill in accordingly:

  • f(0)  =  0
  • f(1)  =  1
  • f(2)  =  f(1) + f(0)  =  1 + 0 = 1
  • f(3)  =  f(2) + f(1)  =  1 + 1 = 2
  • f(4)  =  f(3) + f(2)  =  2 + 1 = 3
  • f(5)  =  f(4) + f(3)  =  3 + 5 = 5
  • f(6)  =  f(5) + f(4)  =  5 + 3 = 8

My point here is that the function notation “does something”. It provides a way of defining and referring to (here, mathematical) concepts.

Less well-known, but appearing in university philosophy courses, is the lozenge symbol, ◊, which means “possible” in a particular kind of logic called modal logic. It seems plausible that there is something meaningful here in Lacan’s use of the symbol too.

Here is Lacan, “explaining” his notation for non-mathematicians:


Lacan doesn’t try to explain what the notion means; he doesn’t seem to want readers to understand. Maybe he is just too clever and if only we persevered we would get what he means. However, elsewhere in the same text Lacan uses arithmetic to argue that “the erectile organ can be equated with √(-1)”. I’m told this is a joke because √(-1) is an imaginary number. Maybe trainee psychoanalysts learn about complex numbers? Maybe all Lacanian discourse is dadaist performance.

Alan Sokal and Jean Bricmont have written a book-length critique of Lacan’s maths and others’ similar use of natural science concepts. Having read lots of mathematical texts and seen how authors make an effort to introduce their notation, I think it’s entirely possible Lacan is a fraud. That might sound harsh, but forget how famous he is and just look at how he writes.

Prover9 and Mace4

Just found two fantastic programs and a GUI for exploring first-order classical models and also automated proof, Prover9 and Mace4.  There are many other theorem provers and model checkers out there.  This one is special as it comes as a self-contained and easy to use package for Windows and Macs.

There are many impressive examples built in which you can play with.  To start easy, I gave it a little syllogism:

all B are A
no B are C

with existential presupposition, which is expressed simply:

exists x a(x).
exists x b(x).
exists x c(x).
all x (b(x) -> a(x)).
all x (b(x) -> -c(x)).

and asked it to find a model. Out popped a model with two individuals, named 0 and 1:

- a(1).

- b(1).

- c(0).

So individual 0 is an A, a B, but not a C. Individual 1 is not an A, nor a B, but is a C.

Then I requested a counterexample to the conclusion no C are A:


- b(1).

- c(0).

The premises are true in this model, but the conclusion is false.

Finally, does the conclusion some A are not C follow from the premises?

2 (exists x b(x)) [assumption].
4 (all x (b(x) -> a(x))) [assumption].
5 (all x (b(x) -> -c(x))) [assumption].
6 (exists x (a(x) & -c(x))) [goal].
7 -a(x) | c(x). [deny(6)].
9 -b(x) | a(x). [clausify(4)].
10 -b(x) | -c(x). [clausify(5)].
11 b(c2). [clausify(2)].
12 c(x) | -b(x). [resolve(7,a,9,b)].
13 -c(c2). [resolve(10,a,11,a)].
16 c(c2). [resolve(12,b,11,a)].
17 $F. [resolve(16,a,13,a)].

Indeed it does. Unfortunately the proofs aren’t very pretty as everything is rewritten in normal forms.  One thing I want to play with is how non-classical logics may be embedded in this system.

It’s funny how the same names keep popping up…

I first heard of Per Martin-Löf through his work in intuitionist logic, which turned out to be important in computer science (see Nordström, Petersson, and Smith, 1990).  His name has popped up again (Martin-Löf, 1973), this time in the context of his conditional likelihood ratio test, apparently used by Item Response Theory folk to assess whether two groups of items test the same ability (see Wainer et al, 1980).  Small world.


Martin-Löf, P. (1973). Statistiska modeller. Anteckningar fran seminarier lasaret 1969–1970 utarbetade av rolf sundberg. Obetydligt ändrat nytryck, october 1973 (photocopied manuscript). Institutet för Säkringsmatematik och Matematisk Statistik vid Stockholms Universitet.

Bengt Nordström, Kent Petersson, and Jan M. Smith. (1990). Programming in Martin-Löf’s Type Theory. Oxford University Press.

Howard Wainer, Anne Morgan and Jan-Eric Gustafsson (1980).  A Review of Estimation Procedures for the Rasch Model with an Eye toward Longish Tests.  Journal of Educational Statistics, 5, 35-64


“… there can hardly be any question that what ‘semantics’ conveyed and conveys to the mind of the general reader is a theory of meaning, which Tarski’s theory most emphatically was not. By calling his theory ‘semantics,’ Tarski opened the door to endless misunderstandings on this point. There has been significant damage to logic arising from such misunderstandings, from confusion of model theory or ‘semantics’ improperly so-called with meaning theory or ‘semantics’ properly so-called.”
—From Tarski’s Tort by John P. Burgess


“This series of lectures on proof-theory is a priori dedicated to mathematicians and computer-scientists, physicists, philosophers and linguists ; and, since we are no longer in the XVI—not to speak of the XVIII—century, it is doomed to failure. […] This being said, plain success is not the only possible goal ; mine might simply be the exposition of a disorder in this apparently well-organised universe, in which logic eventually took its place between two beer mugs and the Reader’s Digest, and does not disturb, no longer disturbs—a sort of fat cat purring on the carpet.”

–Jean-Yves Girard, The Blind Spot