This school, which flourished in the 5th century bce, was distinguished by its radical monism—i.e., its doctrine of the One, according to which all that exists (or is really true) is a static plenum of Being as such, and nothing exists that stands either in contrast or in contradiction to Being.

Among the material monists were the three Milesian philosophers: Thales, who believed that everything was composed of water; Anaximander, who believed it was apeiron; and Anaximenes, who believed it was air.

Parmenides held that the multiplicity of existing things, their changing forms and motion, are but an appearance of a single eternal reality (“Being”), thus giving rise to the Parmenidean principle that “all is one.” From this concept of Being, he went on to say that all claims of change or of non-Being are illogical.

The eleatics were classified as a pre-Socratic philosophy because it came before Socrates was alive and before he developed his ideas. The eleatics ultimately believed that truth can be obtained through logic and clarity, and they preferred to leave their five senses out of their musings.

Christian von Wolff
The term monism was introduced in the 18th century by Christian von Wolff in his work Logic (1728), to designate types of philosophical thought in which the attempt was made to eliminate the dichotomy of body and mind and explain all phenomena by one unifying principle, or as manifestations of a single substance.

3.4. 1.1 Thales of Miletus. The earliest known discovery of electricity comes in 585 BC in a statement written by Thales of Miletus. Thales recognized the attractive powers of amber rubbed with animal fur as well as iron to lodestone.

Marxism has been one of the most influential monistic systems in the Modern Times. Although some Marxist thinkers, were pluralist in their methodology in some fields of inquiry, but Marxist philosophy has been essentially a monistic tradition.

Existence monism targets concrete objects and counts by tokens. This is the doctrine that exactly one concrete object token exists. Priority monism also targets concrete objects but counts by basic tokens.

Various kinds of monism can be distinguished: Priority monism states that all existing things go back to a source that is distinct from them; e.g., in Neoplatonism everything is derived from The One. In this view only one thing is ontologically basic or prior to everything else.

Different types of monism include: Views contrasting with monism are: Metaphysical dualism, which asserts that there are two ultimately irreconcilable substances or realities such as Good and Evil, for example, Manichaeism. Metaphysical pluralism, which asserts three or more fundamental substances or realities.

The wide definition: a philosophy is monistic if it postulates unity of the origin of all things; all existing things return to a source that is distinct from them. The restricted definition: this requires not only unity of origin but also unity of substance and essence.

Monism attributes oneness or singleness (Greek: μόνος) to a concept e.g., existence. Various kinds of monism can be distinguished: Priority monism states that all existing things go back to a source that is distinct from them; e.g., in Neoplatonism everything is derived from The One.