Like René Descartes (1596–1650), Leibniz thought that the basic unit of existence was substance. But whereas Descartes posited two primary kinds of substances—mind and matter—Leibniz posited one immaterial kind of substance, which had many, many instances that he called “monads.” Monads, according to Leibniz, are indivisible units of psychic or mental or spiritual force, each one of which perceived all of the other monads as an aspect of its own inner states.

Each monad had an organic body that “mirrored” what was happening in other monads, but not as a direct effect. That is, like a cell containing all of the chromosomes and genes of the animal of whose body it is a part, for Leibniz each monad contained within itself complete information about the rest of world. In addition, every monad contained its own future states, and of course, within those future states would be the monad’s perception of the future states of every other monad. This world system of monads was created by God and its main feature is the pre-established harmony that results in human perceptions of direct inter-action and inter-relationships.

Monads form colonies and colonies of colonies with dominant monads at different levels of organization. These collections of monads constitute real physical existence. Both space and time are abstractions and not substances. Space, according to Leibniz, is the form of possible coexistences; and time is the form of possible successive existents (things that exist).