Gabriel Seil (1852–1922) is a French historian of philosophy and painting, professor at the Sorbonne, author of several works on aesthetics, sociology and moral philosophy, which had authority at the turn of the 19th and 20th centuries. Based on the entire body of information about Leonardo da Vinci, accessible and systematized by him, the author discusses the contribution of this universal man to science, citing many quotes from the master’s manuscripts. He places the latter in the context of the contemporary noosphere and accompanies it with detailed comments. The main part is preceded by a biography of Leonardo, based on the biography of Giorgio Vasari and other, less well-known sources not available in Russian. A comprehensive study, which has not lost its relevance to this day, is presented in modern spelling. All links to third-party publications have been preserved.