We were speaking of Leonardo da Vinci, omitting all reference to his significance in the world of Art--that is, only of Leonardo the Scholar and the Searcher. Einstein is far from disputing his place in the Valhalla of great minds, but it was clear that he wished to recommend a re-numbering of my list, so that the Italian master would not occupy a position in just the first rank. The problem of Leonardo excited great interest in me, and it deserves the consideration of every one. The further the examination of his writings advances, the more does this problem resolve itself into the question: How much altogether does modern science owe to Leonardo? Nowadays it is declared in all earnestness that he was a painter and a sculptor only by the way, that his chief profession was that of an engineer, and that he was the greatest engineer of all times. This has in turn given rise to the opinion that, as a scientist, he is the light of all ages, and in the abundance of his discoveries he has never been surpassed before or after his own time. As this question had arisen once before, I had come equipped with a little table of facts, hastily drawn from special works to which I had access. According to my scheme, Leonardo was the true discoverer and author of the following things: Law of Conservation of Momentum. Law of Virtual Velocities (before Ubaldi and Galilei). Wave Theory (before Newton). Discovery of the Circulation of the Blood (before Harvey). Laws of Friction (before Coulomb). Law of Pressure for connected Tubes containing Fluid (before Pascal). Action of Pressure on Fluids (before Stevin and Galilei). Laws of Falling Bodies (before Galilei). True interpretation of the twinkling of stars (before Kepler, who, moreover, did not succeed in finding the real explanation). Explanation of the reflected light of the moon (before Kepler). Principle of Least Action (before Galilei). Introduction of the plus and the minus signs into calculations. Definition of kinetic energy from mass and velocity. Theory of Combustion (before Bacon). Explanation of the motion of the sea (before Maury). Explanation of the ascent of fluids in plants (before Hales). Theory of Fossilization (before Palissy). Added to these there are a great number of inventions, in particular those connected with problems of aviation, such as the parachute (before Lenormand), and so forth.