Pluralism, appropriate to its name, is a concept used many different ways in Philosophy (see below). But, in general terms, it is the theory that there is more than one basic substance or principle.

It is contrasted to Monism, which holds that ultimately there is just one kind of substance, and to Dualism, which holds that ultimately there are two kinds of substance in the universe (or, in Philosophy of Mind, that the mind and matter are two separate substances). Arguably, Dualism is a specific case of Pluralism.

Pluralism is also the name of a largely unrelated ancient Greek Pre-Socratic school of philosophy, which includes Anaxagoras, Archelaus (5th Century B.C.) and Empedocles (see the section on the school of Pluralism).

• In Metaphysics:
  Pluralism is the belief that reality consists of many different substances.
• In Philosophy of Mind:
  Pluralism is the belief that there is a plurality of basic substances making up the minds and bodies of humans.
• In Epistemology:
  Pluralism is the claim that there are several conflicting but still true descriptions of the world, and that that no single explanatory system or view of reality can account for all the phenomena of life.
• In Ethics:
  Pluralism is the supposition that there are many independent sources of value and that there is no single truth, even in moral matters.
• In Political Philosophy:
  Pluralism is the acceptance of a multiplicity of groups with competing interests. This is closest to the concept most commonly in general conversational usage.