German philosopher Leibniz published Monadology in 1720, and his brief treatise is situated in the tradition of metaphysical texts. He is largely responding to Descartes (who posited a mind-body dualism). Leibniz's treatise includes 90 precepts and conclusions about matter.
According to Leibniz, all substance is composed of monads, which is indivisible and without parts. These monads are self-propelling, rather than being acted on by an external force. Additionally, no two monads are exactly alike.
According to Leibniz, there are three kinds of monads, 1) created monads, 2) perceiving monads, and 3) rational monads. Created monads might also be called "entelechies," based on the greek words for "perfection" and "self-sufficiency." This is because all monads are self-sufficient and self-contained. Souls, according to Leibniz, are found in perceiving monads. All animals fall into this category. Human beings are rational monads for the fact that they have memory (and so have a sense of consecutive activity). Leibniz acknowledge that animals are equipped with memory to an extent; however, humans (unlike animals) have both memory and reason, which makes us rational monads.
The source of these monads is God, whose argument Leibniz justifies from reason. God's relationship to humans is like an inventor's relationship with his machines. It is through God that we rational monads are permitted to interact with and perceive one another. According to Leibniz, many universes are possible, but the one God directs (our universe) is supremely perfect. In addition to being perfect, God is without limit. He is unique, universal, and necessary. Monads with minds (rational monads) are permitted to understand God and enter a fellowship with him.
Leibniz did not believe in the transmigration of souls (metempsychosis), but he believed that the soul can change the body by degrees. Leibniz concludes his treatise by imagining a City of God, wherein all monads behave with moral as well as natural perfection. It is a type of monarchy with God as the supreme architect and lawgiver.