I am considered to be the mathematician having the most important impact during the transition from the Renaissance period to the modern world.
Not a mathematician, I was a lawyer and a member of the King's Councils for both Henry III and Henry IV.
As part of this Council, I became an expert in deciphering coded enemy messages to the extent that the Spanish claimed I was "in league with the devil."
Doing mathematics as a hobby, I helped establish the use of decimal notation rather than the more complicated sexagesimal system.
My biggest impact was in algebra, where I introduced the idea of symbolic notation--especially differentiating between a vowel for an unknown and a consonant for a known parameter.
Going beyond symbolism, I introduced novel techniques for solving the cubic equation, which eventually become part of the field of symmetric functions.
Francois Viete (1540-1603)