According to the Identity theory also known as Mind-Brain Identity theory, all mental states m1-m5 belong to the same mental model, denominated as “Blue perception”. They are identical to neuronal states n1-n5, which in turn belong to the same neuronal types. The Identity Theory was formulated in the 1950s by U. Place and J. Smart. The two philosophers went from two following assumptions. The philosophical behaviorism concept which attempted to completely replace mental concepts through physicalist terms was incomplete. Not all mental expressions can be defined in physicalist language, such as feelings can be (such as color perception or pain) apparently not fully considered as behavioral dispositions. Two entities can be identical, without being identical in their meaning. Therefore, expressions of mental states as well as expressions of brain states can have different meanings, however still refer to the same phenomenon, thus denoting an identical concept, allowing for a materialist position going beyond the philosophical behaviorism. The dualism of spiritual and corporeal concept of existence is therefore wrong, according to the supporters of this Mind-Brain Theory. The incompleteness of philosophical behaviorism must not be limited to the failure of materialism.

Place and Smart in contrast presented the hypothesis that consciousness or mental states such sensations are identical to brain states. This identity is therefore of an importance of mental terms as it was adopted by a philosophical behaviorism, but simply through an empirical discovery. The systematic development of the theory of identity was an achievement of the 20th century. Even before Smart and Place it was discussed in the context of the Vienna Circle, it was conceived by M. Schlick and continued to be developed in order to be clarified in the course of the 1950s, mainly by Feigl. After the philosophical behaviorism had been considered as not feasible during the 1950s and 1960s and practically and theoretically impossible, the Identity Theory was also expanded further due to the studies of Place and Smart beyond the narrow confines of consciousness and sensations to the field of propositional attitudes. Today the Mind-Brain Theory is most often associated with the thesis that all mental states can be considered identical with brain states.

The Identity Theory was the most important position in the analytical philosophy of mind, during a short period. This theory had affected the field of philosophy in its present form. However, by the late 1960s this concept was rejected by many philosophers again.

The identity theory was confronted from the start with many objections. The Identity Theory was generally understood as a reductionist theory that aimed to lead the mental state to the physical. However, identity is a symmetric relation. Therefore, it was argued that the Identity Theory not only materialized the mental realm, but also attempted to “spiritualize” the brain states that could be ascribed mental properties. The decisive objection however, referred to multiple realization: a mental state can be realized in different beings throughout different brain states. So can pain differ from a specific brain state. People may have pain, even cats and (probably) amphibians. But it is unlikely that all beings are in the same neural state, when they feel pain. In particular, the objection of multiple realization contributed to a rapid loss of popularity of the Identity Theory. Hilary Putnam, who in 1967 brought the objection, also offered the same to an alternative: the functionalism. The different brain states should implement all a functional state which is then identical to the mental state. Functionalism became was for this reason the “orthodoxy” in the philosophy of mind. Recently however, there are new voices calling for a return to Identity Theory.