Augustin Louis Cauchy was one of the most prominent mathematicians of the first half of the nineteenth century. He started the project of formulating and proving the theorems of calculus in a rigorous manner and was thus an early pioneer of analysis. He also gave several important theorems in complex analysis and initiated the study of permutation groups. A profound mathematician, Cauchy exercised by his perspicuous and rigorous methods a great influence over his contemporaries and successors. His writings cover the entire range of mathematics and mathematical physics.

Sourced

• Residues arise ... naturally in several branches of analysis ... . Their consideration provides simple and easy-to-use methods, which are applicable to a large number of diverse questions, and some new results ... .
  • Sur un nouveau genre de calcul, 1826

• ... très souvent les lois particulières déduites par les physiciens d'un grand nombre d'observations ne sont pas rigoureuses, mais approchées.
  • ... very often the laws derived by physicists from a large number of observations are not rigorous, but approximate.

Unsourced

• I am a Christian which means that I believe in the deity of Christ, like Tycho de Brahe, Copernicus, Descartes, Newton, Leibnitz, Pascal… like all great astronomers mathematicians of the past.