Meditations on First Philosophy
Spinoza takes this proposition to follow directly from everything he says prior to it. Spinoza's monism is contrasted with Descartes' dualism and Leibniz's pluralism. Thus, Spinoza avoids the unsolvable problem of how mind and body interact, which troubled Descartes in his "Meditations on First Philosophy." Specifically, how can immaterial mind interface with material body, and vice-versa? They exist in wholly different categories.