In discussing the representation in which the being and activity of the monads consist, we must not think directly of the conscious activity of the human soul.  Representation has in Leibnitz a wider meaning than that usually associated with the word.  The distinction, which has become of the first importance for psychology, between mere representation and conscious representation, or between perception and apperception, may be best explained by the example of the sound of the waves.  The roar which we perceive in the vicinity of the sea-beach is composed of the numerous sounds of the single waves.  Each single sound is of itself too small to be heard; nevertheless it must make an impression on us, if only a small one, since otherwise their total—­as a sum of mere nothings—­could not be heard.  The sensation which the motion of the single wave causes is a weak, confused, unconscious, infinitesimal perception (petite, insensible perception), which must be combined with many similar minute sensations in order to become strong and distinct, or to rise above the threshold of consciousness.  The sound of the single wave is felt, but not distinguished, is perceived, but not apperceived.  These obscure states of unconscious representation, which are present in the mind of man along with states of clear consciousness, make up, in the lowest grade of existence, the whole life of the monad.  There are beings which never rise above the condition of deep sleep or stupor.

In conformity with this more inclusive meaning, perception is defined as the representation of the external in the internal, of multiplicity in unity (representatio multitudinis in unitate).  The representing being, without prejudice to its simplicity, bears in itself a multitude of relations to external things.  What now is the manifold, which is expressed, perceived, or represented, in the unit, the monad?  It is the whole world.  Every monad represents all others in itself, is a concentrated all, the universe in miniature.  Each individual contains an infinity in itself (substantia infinitas actiones simul exercet) and a supreme intelligence, for which every obscure idea would at once become distinct, would be able to read in a single monad the whole universe and its history—­all that is, has been, or will be; for the past has left its traces behind it, and the future will bring nothing not founded in the present:  the monad is freighted with the past and bears the future in its bosom.  Every monad is thus a mirror of the universe,[1] but a living mirror (miror vivant de l’univers), which generates the images of things by its own activity or develops them from inner germs, without experiencing influences from without.  The monad has no windows through which anything could pass in or out, but in its action is dependent only on God and on itself.