Non-biological systems which exhibit the same functional relationships as humans do, such as systems of computer chips, can be said to have the same mental state. As such, mental states are not based on the intrinsic properties of the mental state in question, such as the stuff it is made of. The same state may be shared by things with different physical makeups, thereby distinguishing between the role which a mental state plays, and the occupant in which the state exists. Functionalism may be contrasted both to behaviorism and identity theory in its account of mental events. Behaviorism defines mental events solely in relation to sensory input and behavioral output.
Identity theory restricts mental events to brain activity.
Functionalism, by contrast, acknowledges that mental events may be instantiated in systems or machines other than brains. A pop machine, for example, shows these features insofar as it has instructions on various acceptable inputs with various associated behavioral outputs. Based on this model, Putnam argues that humans are probablistic automatons. A second type of functionalism, defended by David Armstrong and David Lewis, involves a causal theory of mind. Mental states are defined by a common sense understanding of the situations in which they appear and the behavior that is elicited.