Discourse on Metaphysics
13. Since the individual notion of each person includes once and for all everything that will ever happen to him, one sees in it the a priori proofs of the truth of each event, or, why one happened rather than another. But these truths, however certain, are nevertheless contingent, being based on the free will of God or of his creatures, whose choice always has its reasons, which incline without necessitating.
Think about 14, and 15--which motivate the fuller theory of monads.
Mondadology
The ultimate constituents of reality are monads. Monads are mind-like entities--simple (indivisible) substances. Their sole activity is perception or representation, and they change constantly and continuously from one set of perceptions to another, a process which Leibniz calls appetition. Perception and appetition are the only properties that monads have; in particular they are not located in space, though their perceptions do provide the basis for space.
Monads are entirely self-contained. They are “windowless.” They do not interact with anything else. The essential nature of each monad, as specified by their individual complete concepts, includes a specification of all of the perceptions which it will have. Notice that the perceptions of monads are very different from what Descartes thinks our perceptions are--explain the difference.
Each monad represents the entire universe from its unique perspective. While everything is represented, some things are represented more clearly and others more confusedly; each monad perceives more clearly what is closer to it (where closeness is a function of similarity of point of view, not of spatial distance).
There is a maximum degree of continuity and harmony with regard both to the sequence of perceptions within a partiuclar monad and also to the comparative perceptions of diverse monads. Why is there such pre-established harmony? Because this is the best of all possible worlds.
A phenomenon is anything represented in the perceptions of one or more monads. Some phenomenal are mere phenonmena, idiosyncratic perceptions of one or a few monads which have the status of illusion. But other pheomena, those which reflect objective features of the perceptions of all monads, are well-founded---e.g. ordinary objects, people, and other living organisms, causality, time and space.
An ordinary object, like a table, is an aggregate of monads: a collection of monads whose perceptions are closely in agreement so that they appear to each other and to other monads as a unified whole. But such unification, though well-founded, is only apparent, a matter of similarity of perceptions as produced by the pre-established harmony; there is an appearance of causal interaction among the members of such an aggregate, but of course no genuine interaction.
A sentient organism, as opposed to a table, is a structured aggregate of monads, one in which one monad in the aggregate represents the changes of state of all the other monads in the aggregate with an unusually high degree of clarity. The dominanat monad is the mind of the person or animal in question and provides the unifying principle of the organism. In a person, this dominant monad is a spirit. Leibniz often writes as though the presence of such a dominant monad creates a genuine unity among the separate monads of the aggregate, somehow fusing them into an organic unit; but a literal interpreattion of this claim seems incompatible with the main features of his position.