Pierre de Fermat ( August 17 , 1601 â `` January 12 , 1665 ) was a French lawyer at the Parlement of Toulouse , southern France , and a mathematician . Many people see him as the father of modern calculus . Pierre de Fermat ( ; 17 August 1601 or 1607\/8 -- 12 January 1665 ) was a French lawyer at the Parlement of Toulouse , France , and an amateur mathematician who is given credit for early developments that led to infinitesimal calculus , including his adequality .