 [I]t is good to distinguish between perception, which is the internal state of the monad [i.e., simple substance] representing external things, and apperception, which is consciousness, or the reflective knowledge of this internal state, something not given to all souls, nor at all times to a given soul. Moreover, it is because they lack this distinction that the Cartesians have failed, disregarding the perceptions that we do not apperceive, in the same way that people disregard imperceptible bodies. This is also what leads the same Cartesians to believe that only minds are monads, that there are no souls in beasts, still less other principles of life. (“Principles of Nature and Grace” §4, AG 208)