Substance monism is the view that the universe contains only one kind of entity. According to substance monists, the universe is not divided into mental and physical realms, but rather everything is made of one kind of ‘stuff’.
There is a form of monism according to which everything is mental: idealism. According to idealism, what appear to be physical objects are in fact nothing more than mental constructs, ideas. Most substance monists, however, hold that the universe contains only physical or material entities. This position is known either as “physicalism” or “materialism”.
The main challenge for materialists is that of offering a plausible account of mind in purely physical terms. There are several different theories that try to do this, including behaviourism, functionalism, and mind-brain identity theory.
Behaviourism attempts to explain mental states in terms of behaviour. It comes in several varieties. Hard behaviourism holds that mental states are identical with certain behaviour; soft behaviourism holds that mental states are to be understood in terms of behavioural dispositions. Though behaviourism is important historically, neither of its forms is well-regarded by contemporary philosophers.
More plausible than behaviourism is functionalism. Functionalism holds that mental states are defined by their functional role, by the effect that they have on us. Any state that performs the appropriate functional role counts as the mental state. Among other things, this view opens the door to the possibility of creating artificial intelligence.
Mind-Brain Identity Theory
Mind-brain identity theory holds that the mind and the brain are one and the same thing. To be in a particular mental state is nothing more than to be in a particular brain-state. The mind is the brain.