I do not admit what you courteously concede, namely that all my views might hold even if what I wrote about the extension of matter were refuted. It belongs to the main, and to my mind, most certain foundations of my physics. I profess that no answer satisfies me in physics unless it be of the kind that is called logical necessity except, of course, concerning those things that can only be known through experience, such that there is only one Sun or one Moon around the Earth (letter of 5 February 1649, Descartes, AT, V, 275).