Spinoza
Descartes had split the universe into two kinds of stuff (minds and matter) and a God standing outside both. Spinoza looked at that picture and decided the splits were the mistake. There are not two substances and a maker on top of them. There is one thing, infinite, that depends on nothing else to exist, and everything in existence is a passing wrinkle in it. He called that one thing God, or Nature, and he meant the same thing by both words. From that single claim he derived, in the cold steps of geometry, a world with no free will, no personal God listening to prayers, no soul that survives the body, and a kind of blessedness available to a person right now, in this life, by understanding. It got him cast out of his own community at twenty-three and his masterwork banned. It also produced one of the few philosophies that has been called, by serious people, a way to live.
