6 ideas
Imagine a machine that thinks and perceives, enlarged until you could walk inside it like a mill. Inside you would find only parts pushing against one another, never anything that explains a perception. So perception cannot be explained by mechanical arrangement and must be sought in simple substances rather than in composites.
Composite things must ultimately be made of simples that have no parts, no extension, and no shape. Because they have no parts, nothing can enter or leave them: monads are 'windowless' and cannot be causally affected by anything outside. All change in a monad therefore comes from an internal principle, which Leibniz calls appetition, moving it from one perception to the next.
Monads cannot act on one another, and minds and bodies cannot either. They still appear to interact because God set every substance's internal program at creation so that all of them unfold in perfect correspondence, like separate clocks that were synchronized once. What looks like causation is coordinated, independent development.
Each monad perceives the entire universe from its own point of view, the way one city looks different from each vantage point. Monads differ from one another not in what they represent but in how distinctly they represent it.
Leibniz grades mental life in three tiers. Bare perception is an inner state representing multiplicity and can be wholly unconscious, as in a dreamless swoon. This lets unconscious perceptions exist and explains why the Cartesians wrongly equated mind with consciousness.
Nothing is or happens without a sufficient reason why it is so and not otherwise. Among infinitely many possible worlds, God's choosing of one requires a reason, and that reason is fitness or degree of perfection. The actual world is therefore the one with the greatest variety combined with the greatest order.