Rene Descartes set out what he took to be a proof that bodies exist, and proposed to adopt the sceptic's own method: to doubt everything, unless something was found that could not be doubted. One of the first attempts to refute Descartes's proof of a material world was made by Henricus Regius, who rejected some of the metaphysical theses that Descartes believed were the indispensable foundation of his physics. Heterodox Cartesians retreated from Descartes's confidence that the existence of a material world can be proven with certainty. Attacks on Cartesianism poured from the pens of sceptical writers like Simon Foucher, Pierre-Daniel Huet, and Jean Du Hamel. Among other things, they attacked Descartes's purported proof of a material world. Spinoza's reason for asserting the necessary existence of extension is that God necessarily exists and extension is necessarily an attribute of God.