## 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: Huh?

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(0).
- a(1).

b(0).
- b(1).

- c(0).
c(1).```

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:

```a(0).
a(1).

b(0).
- b(1).

- c(0).
c(1).```

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.

References

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

## “Semantics”

“… 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

## Success

“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