ere would be no means of perceiving any change in things. Whatever is in a compound can come into it only through its simple elements and the monads, if they were without qualities (since they do not differ at all in quantity) would be indistinguishable one from another. For instance, if we imagine a plenum or completely filled space, where each part receives only the equivalent of its own previous motion, one state of things would not be distinguishable from another.
9. Each monad, indeed, must be different from every other monad. For there are never in nature two beings which are exactly alike, and in which it is not possible to find a difference either internal or based on an intrinsic property.
10. I assume it as admitted that every created being, and consequently the created monad, is subject to change, and indeed that this change is continuous in each.
11. It follows from what has just been said, that the natural changes of the monad come from an internal principle, because an external cause can have no influence on its inner being.
12. Now besides this principle of change there must also be in the monad a variety which changes. This variety constitutes, so to speak, the specific nature and the variety of the simple substances.
13. This variety must involve a multiplicity in the unity or in that which is simple. For since every natural change takes place by degrees, there must be something which changes and something which remains unchanged, and consequently there must be in the simple substance a plurality of conditions and relations, even though it has no parts.
14. The passing condition which involves and represents a multiplicity in the unity, or in the simple substance, is nothing else than what is called perception. This should be carefully distinguished from apperception or consciousness, as will appear in what follows. In this matter the Cartesians have fallen into a serious error, in that they deny the existence of those perceptions of which we are not conscious. It is this also which has led them to believe that spirits alone are monads and that there are no souls of animals or other entelechies, and it has led them to make the common confusion between a protracted period of unconsciousness and actual death. They have thus adopted the Scholastic error that souls can exist entirely separated from bodies, and have even confirmed ill-balanced minds in the belief that souls are mortal.
15. The action of the internal principle which brings about the change or the passing from one perception to another may be called appetition. It is true that the desire (l'appetit) is not always able to attain to the whole of the perception which it strives for, but it always attains a portion of it and reaches new perceptions.
16. We, ourselves, experience a multiplicity in a simple substance, when we find that the most trifling thought of which we are conscious involves a variety in the object. Therefore all those who acknowledge that the soul is a simple substance ought to grant this multiplicity in the monad, and Monsieur Bayle should have found no difficulty in it, as he has done in his Dictionary, article Rorarius.
17. It must be confessed, however, that perception, and that which depends upon it, are inexplicable by mechanical causes, that is to say, by figures and motions. Supposing that there were a machine whose structure produced thought, sensation, and perception, we could conceive of it as increased in size with the same proportions until one was able to enter into its interior, as he would into a mill. Now, on going into it he would find only pieces working upon one another, but never would he find anything to explain perception. It is accordingly in the simple substance, and not in the compound nor in a machine that the perception is to be sought. Furthermore, there is nothing besides perceptions and their changes to be found in the simple substance. And it is in these alone that all the internal activities of the simple substance can consist.
18. All simple substances or created monads may be called entelechies