Reality philosophy is a branch of philosophy that deals with the nature of reality itself and the relationship between the mind, language, and culture, and reality. Philosophers commonly define reality as a substance that actually exists in an external world. Reality is the independent nature and existence of everything knowable, whether it is knowable by logical inference, empirical observation, or some other form of experience. In discussing the nature of reality, we must distinguish between physical reality and immaterial (non-physical) reality. Physical reality is that which is constrained by physics or physical laws, while immaterial reality is not constrained by physical laws. Reality philosophy also concerns the ways in which reality is, or is not, in some way dependent upon mental and cultural factors such as perceptions, beliefs, and other mental states, as well as cultural artifacts, such as religions and political movements.