Monism attributes oneness or singleness (Greek: μόνος) to a concept, such as to 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.[1] In this view only the One is ontologically fundamental or prior to everything else.
Existence monism posits that, strictly speaking, there exists only a single thing, the universe, which can only be artificially and arbitrarily divided into many things.[2]
Substance monism asserts that a variety of existing things can be explained in terms of a single reality or substance.[3] Substance monism posits that only one kind of substance exists, although many things may be made up of this substance, e.g., matter or mind.
Dual-aspect monism is the view that the mental and the physical are two aspects of, or perspectives on, the same substance.
Neutral monism believes the fundamental nature of reality to be neither mental nor physical; in other words it is "neutral".
^Strawson, G. (2014 in press): "Nietzsche's metaphysics?". In: Dries, M. & Kail, P. (eds): "Nietzsche on Mind and Nature". Oxford University Press. PDF of draft