Fermat is easily one of the best known mathematicians of all time. We all know about Fermat's Last Theorem, Fermat's Little Theorem, his quadrature rule, his invention of probability theory, etc. Every introduction of Fermat, however, mentions that mathematics wasn't his "day job" and that he was a lawyer.

As much as I search, I cannot seem to find anything on his work as a lawyer. Unless his job was divine mathematics as a lawyer, I get the feeling that his work as a lawyer was either unremarkable or just forgotten.

Does anyone have a reference that they can point me to that could describe his role in law? Or something that would tell me what his job would have been?


Fermat wasn't so much a "lawyer" as a magistrat which means that he sat on successively higher levels of the Parlement of Toulouse, France. This period (17th century) was before the emergence of the doctrine of the separation of powers in Western thought, so that the Parlement was not a "Parliament" in the modern English sense of the word, but rather the judicial, legislative, and executive branches rolled into one.

Fermat was extremely active as a magistrat and penned numerous legal documents. In fact he was busy writing one only a few days before he died.

One of the most controversial cases Fermat was involved in concerned the trial of the priest Delpoy and his eventual execution. Fermat himself was unconvinced of Delpoy's guilt but was apparently overruled by his more powerful peers on the Parlement, including the president of the Parlement who is considered to have been rather hostile to Fermat. Fermat is said to have been in shock over the verdict and execution to such an extent that his productivity at the court declined sharply. He was similarly unable to work on mathematics for a period of a few months, as claimed by Digby.

More details on the background of Fermat's legal activity can be found in this 2018 publication in Foundations of Science where you will also find additional references on Fermat's activity at the Parlement.

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    $\begingroup$ Thank you for such a thorough answer. I suppose my question was timely with the publication of this 2018 paper. $\endgroup$ – Joel Jan 28 '18 at 16:25
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    $\begingroup$ The article is currently in "online first" status. I included the date "2018" as an estimated year for inclusion in an issue. It is quite possible that the article will actually appear in a 2019 issue. $\endgroup$ – Mikhail Katz Jan 29 '18 at 9:52
  • $\begingroup$ The Delpoy affair is fascinating. The only info I could find was in Klaus Barner, "Fermat et l’affaire Delpoy." A Huguenot named Lacombe converted to Catholicism and sent his daughter Françoise, aged 13-14, to a distant convent (?). Françoise ran away, and Raymond Delpoy, a priest and Françoise's confessor, took her away and left her with Dr. Pierre Delpoy, probably his brother. If Françoise had converted but then become an apostate, she would be subject to burning at the stake. Barner speculates that the father wanted to accuse the Delpoys of sexual misconduct in order to save his daughter. $\endgroup$ – Ben Crowell May 14 '20 at 13:12

Read Klaus Barner's article Pierre de Fermat (1601? – 1665): His life beside mathematics (pages 12–16).

  • $\begingroup$ That's a great article. Thank you for the reference! $\endgroup$ – Joel Jan 28 '18 at 16:24

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