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Michael Detlefsen

Questioning Truth

michael detlefsen

© S. Godefroy/CNRS Photothèque

The American Michael Detlefsen is a humanist in search of truth. Yet at the same time, he questions the very existence of truth: can several truths, and thus several proofs, co-exist at one and the same time? Surprisingly, or not, Detlefsen tackles these complex questions through mathematics. The philosopher and mathematician from Notre Dame University (Indiana, US) has set up camp in France until 2011 in CNRS-affiliated laboratories, where he heads a program called “Ideals of proof.”
The program explores what are known as “imaginary” numbers, which can result in surprising outcomes. With imaginary numbers, the square of a number (the multiplication of a number by itself) can become negative. “The very existence of these numbers is a totally irrational concept for many people untrained in mathematics.” And yet, this concept is a mathematical truth for those who deal with these numbers on a daily basis.
This questioning of truth has its origins in Detlefsen's own roots, and in the incredible epic of his great-grandfather and great great uncle who decided, at ages 9 and 11, to leave Denmark and their family on their own to embrace the American Dream. “A long and trying adventure then ensued which led them, by boat, train, and then finally on foot, to the town of Dannebrog in Nebraska,” explains Detlefsen. “There, the government gave them land on the condition that they cultivate it for at least five years. The dream became reality.” Detlefsen, who knows the story by heart, can himself raise a number of questions: Why did these children really decide to leave their family? Is the truth, as told to him by his older brother, the real truth? Does a single truth ever exist, or is truth affected by our own history? And what about our own history in relation to History?
When he started his undergraduate studies, Detlefsen turned towards the sciences, mathematics, physics, as well as towards philosophy. Eventually he obtained a PhD in logic. After a dissertation on Gödel's second incompleteness theorem—which shows that certain sentences expressing the consistency of systems are impossible to demonstrate in those systems—the scientist's first position was as professor of philosophy in Minnesota followed by a second at the University of Notre Dame in Indiana. “Do mathematical concepts really exist in nature, or are they only a creation of the mind, which tends to rationalize the order of the world in this manner and project its truth?” That was the starting point of a thought process that Detlefsen has followed throughout his career in the US and across the world.
Croatia, Germany, Switzerland, Australia, Japan, Spain, and many other countries have hosted Detlefsen over the last 25 years. It is now France's turn, where Detlefsen, as a world specialist in the foundations of mathematics, has been awarded the 2007-2011 Senior Chair of Excellence.1 At the end of 2007, he joined a Nancy-based laboratory specialized in the history of sciences and philosophy.2 This year, he signed on with the history and philosophy of science department of Paris-VII University3 for a duration of two years. He will then return to Nancy for a final year before heading back to Notre Dame. Will this take him closer to the truth? That's another question entirely.

Séverine Duparcq

Notes :

1. With the support of CNRS, Paris-VII and Nancy-II Universities, the Collège de France, and Notre Dame University.
2. Laboratoire d'histoire des sciences et de philosophie-Archives Henri-Poincaré (CNRS / INPL Nancy / Universités Nancy-I and -II).
3. Département d'histoire et philosophie des sciences (Université Paris-VII). This includes three structures, two of which are joint research units linked to CNRS.

Contacts :

Michael Detlefsen,
Université Paris-VII, Paris.


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