Pluralism is a Greek Pre-Socratic school of philosophy of the 5th Century B.C., consisting of three major philosophers: Anaxagoras, Archelaus (5th Century B.C.) and Empedocles.
In general terms, they attempted to reconcile the complete rejection of change by Parmenides and the Eleatic School, which generally speaking they accepted, with the apparently changing world of sense experience (things like birth and death and creation and destruction), and thereby find the basis for all change.
The Ionian philosopher Anaxagoras believed that all things have existed from the beginning as an endless number of infinitesimally small fragments of themselves, but in a confused and indistinguishable form. The segregation of like from unlike was carried out by a pure and independent thing called "Nous" (mind), which also causes all motion. Some of his ideas presaged the later development of Atomism.
Archelaus, a student of Anaxagoras, asserted that air and infinity are the principles of all things, that primitive Matter is air mingled with Mind, and that the principle of motion was the separation of hot from cold, from which he endeavoured to explain the formation of the Earth and the creation of animals and humans.
Empedocles was a citizen of Agrigentum, a Greek colony in Sicily, and is best known for being the origin of the cosmogenic theory of the four classical elements (fire, air, water and earth) which he held to be simple, eternal and unalterable, and which are eternally mixed and separated by two divine powers, Love and Strife. Like the Eleatics, he held that it is not possible for something to come into existence from a non-existence, or vice versa, only that original materials are combined and recombined. Empedocles was also influenced by Pythagoreanism in his support for the doctrine of reincarnation.
Pluralism as a philosophical doctrine is a concept used in many different ways, but, in general terms, it is the theory that there is more than one basic substance or principle, whether it be the constitution of the universe, of the mind and body, the sources of truth, etc (see the section on the doctrine of Pluralism).