Theories have been composed and expounded by various philosophers to explain their understanding of the mind. Dualism, behaviorism, and identity theory are well-known theories supported by well-written explanations. A modern theory, functionalism, has been proposed which provides excellent insight into the main problem facing philosophers, the mind/body problem. Functionalism was developed as a combination of behaviorist theory and identity theory. Behaviorism considers being in a mental state to be the same as a physical state, which is an observable behavioral characteristic. For example, if a person reports feeling unhappy, their physical state might include a sullen face or poor posture. On the other hand, identity theory suggests when you experience something; there is a corresponding neurological condition. The error of both theories is that behaviorism fails to incorporate mental states but attempts to do so by saying that the mental state is the observable behavior (Southwell). However, different behaviors can arise from the same stimuli and different stimuli can trigger the same reaction. Identity theory is incorrect because it claims that mental and brain states are identical, but this claim is probably false (Garth). Functionalism includes elements of both theories to clarify reasons and draws on both ideas. The functionalism view can be defined as mental states established by their functional role; that is, mental states and their relationships to other mental states, sensory inputs, and behavioral outputs (Levin). In the article “Functionalism” by Ned Block, he illustrates this concept through a machine by detecting an even or odd number of 1. This machine has two states, S1 and S2; a......middle of article......y.Works CitedLevin, Janet, "Functionalism", The Stanford Encyclopedia of Philosophy (Summer 2010 edition), Edward N. Zalta (ed.), URL = . Southwell, Gareth. “Philosophy of Mind – Behaviorism – Introduction.” Resources, reviews, discussions, and books for students and general readers interested in philosophy - Philosophy Online. 2000. Internet. November 17, 2011. .Chalmers, David. “Absent Qualia, Discolored Qualia, Dancing Qualia.” David Chalmers. Conscious experience. Internet. November 17, 2011. Cole, David, "The Chinese Room Argument", The Stanford Encyclopedia of Philosophy (Winter 2009 edition), Edward N. Zalta (ed.), URL = .