Materialism is a form of philosophical monism which holds that matter is the fundamental substance in nature, and that all things, including mental states and consciousness, are results of material interactions of material things. According to philosophical materialism, mind and consciousness are caused by physical processes, such as the neurochemistry of the human brain and nervous system, without which they cannot exist. Materialism directly contrasts with idealism, according to which consciousness is the fundamental substance of nature.

Monism attributes oneness or singleness 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. In this view only the One is ontologically basic 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. Substance monism asserts that a variety of existing things can be explained in terms of a single reality or substance. 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".