The Monadology

"The Monadology (Monadologie, 1714) is one of Gottfried Leibniz’s works that best define his philosophy, monadism. Written toward the end of his life in order to support a metaphysics of simple substances, the Monadology is thus about formal atoms which are not physical but metaphysical."

Source: http://en.wikipedia.org/wiki/Monadology