Karl Marx was a fiery revolutionary theorist who heralded the imminent demise of capitalism, while Spinoza was a contemplative philosopher who preached rational understanding and voiced skepticism about open rebellion. Spinoza criticized all teleological ideas as anthropomorphic fantasies, while Marxism came to be associated expressly with teleological historical development. Why, then, were socialists of the German nineteenth century consistently drawn to Spinoza as their philosophical guide? Tracie Matysik shows how the metaphorical meeting of Spinoza and Marx arose out of an intellectual conundrum around the meaning of activity. How is it, exactly, that humans can be fully determined creatures but also able to change their world? To address this paradox, many revolutionary theorists came to think of activity in the sense of Spinoza--as relating. Matysik follows these Spinozist-socialist intellectual experiments as they unfolded across the nineteenth century, drawing lessons from them that will be meaningful for the contemporary world.