How does multiple realizability challenge the idea that every mental state is one specific brain state?

The idea that every mental state is identical to one specific brain state is known as type-identity theory. This view suggests that a specific mental phenomenon, such as pain, must always correspond to the exact same type of neurophysiological state.12

Multiple realizability challenges this by arguing that the same mental state can be "realized" or implemented by different physical systems. For example, the same type of pain could be experienced by a human, an animal, or potentially an extraterrestrial, even if their brains are composed of entirely different materials or structures.123

Because it is implausible to require that all these diverse beings share the exact same neurophysiological state to experience the same mental state, multiple realizability demonstrates that mental states cannot be strictly reduced to a single, universal type of brain state. This insight led Hilary Putnam to develop functionalism, which defines mental states by their causal roles—such as their inputs and outputs—rather than by the specific physical hardware that performs those functions.321