8. Nevertheless Monads must have qualities—otherwise they would not even be entities; and if simple substances did not differ in their qualities, there would be no means by which we could become aware of the changes of things, since all that is in compound bodies is derived from simple ingredients, and Monads, being without qualities, would be indistinguishable one from another, seeing also they do not differ in quantity. Consequently, a _plenum_ being supposed, each place could in any movement receive only the just equivalent of what it had had before, and one state of things would be indistinguishable from another. 9. Moreover, each Monad must differ from every other, for there are never two beings in nature perfectly alike, and in which it is impossible to find an internal difference, or one founded on some intrinsic denomination. 10. I take it for granted, furthermore, that every created being is subject to change—consequently the created Monad; and likewise that this change is continual in each. 11. It follows, from what we have now said, that the natural changes of Monads proceed from an internal principle, since no external cause can influence the interior. 12. But, besides the principle of change, there must also be a detail of changes, embracing, so to speak, the specification and the variety of the simple substances. 13. This detail must involve multitude in unity or in simplicity: for as all natural changes proceed by degrees, something changes and something remains, and consequently there must be in the simple substance a plurality of affections and relations, although there are no parts. 14. This shifting state, which involves and represents multitude in unity, or in the simple substance, is nothing else than what we call Perception, which must be carefully distinguished from _apperception_, or consciousness, as will appear in the sequel. Here it is that the Cartesians have especially failed, making no account of those perceptions of which we are not conscious. It is this that has led them to suppose that spirits are the only Monads, and that there are no souls of brutes or other Entelechies. It is owing to this that they have vulgarly confounded protracted torpor with actual death, and have fallen in with the scholastic prejudice, which believes in souls entirely separate. Hence, also, ill affected minds have been confirmed in the opinion that the soul is mortal. 15. The action of the internal principle which causes the change, or the passage from one perception to another, may be called Appetition. It is true, the desire cannot always completely attain to every perception to which it tends, but it always attains to something thereof, and arrives at new perceptions.