Pierre Fermat: Ending the Tyranny of Descartes and the Cartesians.

The article reviews *The Battle for Light: Fermat vs. Descartes, A Sourcebook on Pierre de Fermat's Principle of Least Time*, edited and translated by Jason Ross, which presents the first comprehensive English translation of Pierre de Fermat's correspondence on light and his disputes with René Descartes. It highlights Fermat's experimental method and his principle of least time, contrasting it with Descartes's deductive approach, and traces how Fermat's work influenced later scientists such as Christiaan Huygens and Gottfried Leibniz, who developed the wave theory of light and differential calculus, respectively. The review also discusses the broader scientific context of 17th-century France, emphasizing the intellectual courage required to challenge Cartesian dogma and the limitations of both deduction and induction as scientific methods. Finally, it critiques Isaac Newton's approach to calculus and optics, arguing that Fermat and his successors' hypothesis-driven experimental method better advanced scientific understanding.