“Ah, you see that I’m not telling you cracks!” He was looking for his phone, he came across an A4 sheet blackened with formulas scribbled in a mechanical pencil. Thierry Lhermitte, the famous actor of the eternal Splendid, finds maths sensational. It took him in high school; it took over later, in the early days of computing. Today, this devourer of scientific works and sponsor of the Foundation for Medical Research continues to rub his brain with the challenges of the Mathematical goddess. For L’Express, he agreed to come back to what he likes in this discipline sometimes so badly judged, to dig into what worries him in the drop in level in France, and to give some advice for adults and children. Interview-advocacy of a fervent defender of rationality.
L’Express: “Only maths interested me”, you say of your high school years in Neuilly, in the company of Gérard Jugnot, Christian Clavier and Michel Blanc…
Thierry Lhermitte: I wasn’t interested in much. I still can’t explain it to myself, because today I’m curious about everything. But the one thing that didn’t require work was math. In this matter, when we have understood something, we move forward. There’s no need to cramme. What matters is to understand. It amused me, and fascinated me too. As soon as a problem is posed to me, it obsesses me. Maths allowed me to get my baccalaureate C. It was not won! [Rires.]
When did it become a passion?
It came back to me with the beginning of computing. I got into programming in the 1980s. What I liked was dissecting the stages of reasoning to make it reproducible: breaking a big problem into small problems, then turning it into an algorithm that breaks down the way arrive at the solution. At that time, many “dunces” came back to science thanks to that. IT did not require any culture, just to think. I loved it, and started reading math books – I have a huge collection of Odile Jacob! One of the first books I immersed myself in was Randomly by Ivar Ekeland on the notion of chance in mathematics. That’s when I discovered that computers couldn’t create a semblance of chance.
I also really liked Stella Baruk’s books, such Captain’s age, in which this specialist in pedagogy digs into the reasons for the confusion of many students vis-à-vis maths. She explains in particular that the words do not mean the same thing depending on whether they are used in everyday language or for mathematics. For example, in everyday life, a “function” is a job. In mathematics, it is a transformation. This confusion does not facilitate learning. Moreover, according to Stella Baruk, students are taught to perform operations, without them understanding what it is for. We simply teach how to use the tool, without the why and how.
“What’s Pi doing in there?”
Do you exercise?
I was contacted by a math teacher who lives near where I ride. He sends me math problems to solve. I have other things to do in life, but, as I’m a little obsessive, I always end up coming back to them. The other time, it was funny, he asked me how much is 1 + 2 + 3 +… until not. Legend has it that Gauss found the answer at age 7 (at least when it comes to adding from 1 to 100). The technique is simple: visualize on one line the numbers from 1 to 100, then on the line below the same numbers in descending order, from 100 to 1. Then, we add the two lines: 1 + 100 = 101, 2 + 99 = 101, 3 + 98 = 101… up to 100 + 1 = 101. The answer is therefore the number of pairs multiplied by the sum of each pair, all divided by 2. In this case : 100 x 101 / 2 = 5050. Or, more generally: not (not + 1) / 2. By putting children on the path, they can make it happen.
Much more complex: the sum of 1/1² + 1/2² + 1/3² up to 1/not² equals π²/6. It’s fascinating: what is Pi doing in there?! I’m not developing you, huh, I understood at the time, and now I’ll tell you that I’m not so sure anymore… [Rires.] Sometimes, in math, we have epiphanies, we understand reasoning, it’s quite beautiful.
The level of France in this subject continues to decline in international rankings such as TIMSS. Does this worry you?
Mathematics is essential, even in the life sciences. I was in a lab that measures the toxic particles we have in our bodies. The study looked at meconium (first stool) and breast milk. These are minimal quantities, which must be weighed. Scientists are able, with mass spectrometers, to determine each type of toxic molecule by its weight. The unit is the daltons, or 1.6 to the power -24 grams. It’s very small, it’s only measurable with math! The same goes for epidemiological models: only mathematics can make it possible to draw solid conclusions from the multiple data at stake. Without that, it would be a wet finger. You need good mathematicians because math is everywhere.
But, beyond that, I would say that there is also a worrying effect of the drop in level on “the man in the street”: the decline of logic, of orders of magnitude, etc., it is a fuel for credulity. If we don’t learn the mechanism of mathematical reasoning or the logic of formulas, then all formulas are equal. There is no longer a path of thought that is logical or not. Everything is worth it. There is only what pleases or displeases me. We pass from knowledge to belief. It is the “denial of rationality” described by the sociologist Gérald Bronner, for example.
Your environment does not always shine in this area. Juliette Binoche or Sophie Marceau were able to relay conspiratorial remarks on vaccines…
In contrast, Anouk Grinberg did a show on mathematician Alexandre Grothendieck. And Alexandre Astier is a science buff. There are strong supporters of reason and science, too. But it is true that we are more accustomed to hearing nonsense…
“The conspirators are the useful idiots of the real conspirators”
Has the antivax movement worried you?
I was appalled. We must distinguish rational doubt from pathological doubt, which casts suspicion on everything. You sat down on a chair when you arrived: who told you that the chair wasn’t going to break, and you, you break your coccyx? [Rires.] I have a little theory about the conspirators: I think they are the useful idiots of the real conspirators. Pathological doubt, applied to everything, drowns out the things that really deserve to be pointed out.
The Mediator, for example: it’s a real health scandal, it killed people. But if you start seeing health scandals behind every box of aspirin, then the Mediator finds itself drowning in systematized doubt. The filthiness of the tobacco or sugar industries is the fruit of real conspirators. But it suits them that we drown their misdeeds in absurd and generalized things. This is also what Russian trolls do.
The last time you were interviewed, you were reading a book on Bayesian inference…
It is not easy to popularize. These are conditional probabilities: this makes it possible to model the probability that an event A will occur, knowing that an event B has already occurred (or not). Knowing B, what is the probability of A? In The knowledge formula, Lê Nguyên Hoang shows that our brain probably works like this. You can tell yourself that a chair you have to sit on is going to break. But the probability that the chair does not break evolves as our experience progresses. Experience reinforces or diminishes our perceptions of risk.
You would also have dreamed of making a film about Alan Turing…
I read his biography, Alan Turing or the enigma of intelligence by Andrew Hodges over twenty years ago. I had tried to buy the rights, but they were already taken. What ultimately led to the movie Imitation Game with Benedict Cumberbatch. Turing’s story is incredibly romantic. The guy managed to create a programmable machine in which you can change the operations, with just 0s and 1s. Already, it’s extraordinary. But in addition it was done at Bletchley Park, in the middle of World War II, to decode the Nazi encryption machine, Enigma. Finally, there is his sad fate after the war: the impossibility of promoting his work due to secrecy, then the conviction for homosexuality, and his suicide with a poisoned apple.
“We must teach cognitive biases at school”
In the UK, Turing is now featured on 50 pound notes. Isn’t it the responsibility of nations to keep mathematical culture alive? Previously, we had Pascal on our tickets…
Fortunately, there are still figures, like Cédric Villani, who remain known to the general public. But how long has it been since a scientist or a professor of medicine made the cover of Paris Match?
There was Didier Raoult…
Yes. [Soupir.] There, he left, issuing a warning cry against an epidemic of lice coming from Ukraine. Raoult says he is the world’s best specialist on the subject, obviously. It’s crazy… Before, scientists were heroes. As we have seen during the Covid-19 period, there is now a pathological doubt about any doctor.
What do you recommend to get young people interested in math?
I already recommend the website The Logical Island. They are the Deschians, but in math! In this duo of clowns, one plays dumb, and the other explains concepts of mathematics, logic or critical thinking. I saw them at the Institut Poincaré, it’s for all levels, and it’s absolutely wonderful.
Otherwise, I always ask the children how many squares there are on the tablecloth. They tell me that there are many. I explain to them that it is enough to multiply the number of squares on the width and the length. They tell me “Ah, that’s what it’s for!” I also like to explain to them that equalities are like a scale. You have to understand the logic of the operations, and not just apply them with a calculator.
And what books do you recommend for adults?
Logicomix, a comic on the history of the foundations of mathematics. Still in comics An endless world by Jean-Marc Jancovici, illustrated by Christophe Blain. This is an exciting way to reflect on climate and energy issues, and stop talking nonsense. And then I recommend System 1 / System 2 by Daniel Kahneman, who describes our two thought systems, one fast, intuitive and emotional, the other slower and more thoughtful and logical. Kahneman has shown how much you have to be wary of your own judgments. It also seems desirable to me that the main cognitive biases be taught at school. For example, students should be made aware of confirmation bias, which consists of favoring information that confirms preconceived ideas and therefore beliefs, or negativity bias, which means that we are marked more by bad news than by good. Our brain always pays more attention to singular phenomena than to things that work but that we see all the time. The famous “trains that arrive on time”.