Pierre de Fermat developed around 1636 a method of "adequality" to find maxima, minima, and tangents to curves that anticipates fundamental features of differential calculus. His technique introduces an infinitesimal variation, evaluates the resulting expression, and equates the two states — a procedure that Leibniz knew and that influenced his own development of calculus. The great Newton-Leibniz dispute retrospectively overshadowed Fermat's role, relegating him to a mere footnote in most historical narratives.