Summary
Simple substances
Leibniz begins from the existence of compounds. Because a compound is an aggregate, he argues that there must be simple substances from which it is composed. He calls these simples monads. A monad has no parts, extension or shape and therefore cannot be divided or naturally assembled and dissolved as bodies can. Creation and annihilation, rather than ordinary physical processes, mark the only possible beginning and end of a simple substance.
Internal change and perception
Although monads have no parts, they are not inert. Each possesses qualities and passes continuously through changing internal states. Leibniz calls the representation of multiplicity within a unity perception, distinguishing it from conscious awareness. Appetition is the internal tendency that carries a monad from one perception towards another. Change therefore arises from the monad’s own principle rather than from something physically entering it.
Degrees of mental life
Monads differ through the clarity of their perceptions. Bare monads possess confused perception; animal souls have sensation and memory; rational souls can reflect, know necessary truths and form the idea of the self. Sleep or unconsciousness does not remove all perception. It temporarily removes distinct awareness, just as countless small impressions may combine without being separately noticed.
Reason and sufficient reason
Human reasoning rests upon the principles of contradiction and sufficient reason. Necessary truths are established when their denial produces contradiction. Truths of fact are contingent, yet still require an explanation of why this state of affairs exists rather than another. Because the chain of contingent explanations cannot supply its own final ground, Leibniz argues that it points to a necessary substance.
God and possible worlds
God is presented as the necessary being and source of actuality and possibility. Divine understanding contains the possible orders of things, while wisdom selects the world possessing the greatest achievable combination of variety and order. The claim that this is the best possible world does not mean that every local event appears good in isolation. It means that the complete system is chosen according to an overall principle of perfection.
Perspectives upon one universe
Every monad represents the whole universe from its own point of view. Monads therefore differ not because each inhabits a separate world, but because each expresses the same world with a different degree and distribution of clarity. Leibniz compares this to one town viewed from many sides. The perspectives vary while the order represented remains one.
Pre-established harmony
Monads have no windows through which causal influence can pass. The apparent interaction between mind and body, and among created things generally, is explained by a harmony established in their original natures. Each unfolds according to its own law, yet its states correspond with the states of others. Bodily events and perceptions agree without one needing to transmit a physical item into the other.
Living bodies and the moral world
Organic bodies are machines whose parts remain organised to infinity, unlike artefacts whose smallest pieces cease to function as machines. Souls remain connected with changing organic bodies through gradual transformation rather than complete separation. Rational spirits add knowledge of God and moral relationship, forming a community in which natural order and justice are ultimately coordinated.