Monism is the strange and beautiful idea that the cosmos is the source of all being. Historically, this view had its most influential exponent in Spinoza, who held that there was only one substance in the world, which he called 'God or nature'. Twentieth century English speaking philosophers poured scorn on monism, taking it to be unscientific and contrary to common sense. However, a number of leading contemporary philosophers have been drawn to the view, believing it to be not only consistent with common sense, but resonant with contemporary science. The first half of this volume explores the contemporary flowering of monism, bringing together its defenders and detractors in a lively debate. The second half of the volume explores the roots of monism in Spinoza, and reassesses what exactly Spinoza meant when he claimed that there is only one substance.