The thesis that all of reality is of one kind. See materialism, idealism, neutral monism, dualism.

In philosophy of mind, monism is usually contrasted with the dualist position that mind and matter are deeply different. Thus, monism is the claim that mind and matter are essentially the same. However, this 'sameness' has come in a number of different and contradictory varieties. For example, Hobbes felt that the mental is merely and epiphenomena of the physical, thus the physical is the one real substance (Contemporary materialism is also a form of physicalistic monism (see Churchland, 1996). In direct contrast, Berkeley postulated that the physical is just a collection of ideas (hence, idealism) and thus the mental is the only thing that really exists. Finally, there are a number of positions similar to Spinoza's property dualism, often referred to as dual-aspect theory. Spinoza held a position in which the mental and the physical are simply two modes of a more basic substance (it should be noted that strictly speaking, Spinoza was not a property dualist as he held that the mental and the physical were two of a possible infinite number of modes of the basic substance, nevertheless he is typically labeled as one). For Spinoza, this basic substance was God. Thus the only real thing is God, who is neither physical nor mental. Spinoza's position is similar to that of Russell's neutral monism, however the latter is not committed to the belief that a supreme being is the more basic substance.
Chris Eliasmith