In his later work, La Nouvelle monadologie (1899), he maintains that each monad is a simple substance, endowed with representation, which is consciousness in form, phenomenon in matter as represented.
Thus, the atom of hydrogen is a monad simple radical, the atom of oxygen a dyad simple radical, whilst the group OH is a monad compound radical.
Each monad is an original independent being, and is determined to take this particular point in the universe, this place in the scale of beings.
Such mentally endowed substances might be called souls; but, as he distinguished between perception and apperception or consciousness, and considered that perceptions are often unconscious, he preferred to divide monads into unconscious entelechies of inorganic bodies, sentient souls of animals, and rational souls, or spirits, of men; while he further concluded that all these are derivative monads created by God, the monad of monads.
Each monad works out necessary results, but these flow from its own nature; and so in a sense it is free.
That in it first and in it alone this condition is realized - the individual soul must be held to be an ultimate reality reflecting in its inmost nature, like the monad of Leibniz, the complete fulness and harmony of the whole.
The more Spinozistic side of Leibnitz's thought - God as Monad of Monads - is a theistic postulate if hardly a theistic proof.
Those elements which are equivalent in combining or displacing power to a single atom of hydrogen are said to be univalent or monad elements; whilst those which are equivalent to two atoms of hydrogen are termed bivalent or dyad elements; and those equivalent to three, four, five or six atoms of hydrogen triad, tetrad, pentad or hexad elements.
Leibniz, on the other hand, regarded his monad as the ultimate element of everything.
For the soul, by its nature as a single monad indestructible and, therefore, immortal, death meant only the loss of the monads constituting the body and its return to the pre-existent state.