Functionalism Philosophy of Mind

In the philosophy of mind, functionalism is the thesis that mental states are constituted solely by their functional roles, rather than by their internal constitution or physical substrate. A mental state such as believing, desiring, or feeling pain is defined by the causal relations it bears to sensory inputs, other mental states, and behavioural outputs. This approach emerged in the mid-twentieth century as a prominent alternative to earlier theories such as Cartesian dualism, behaviourism, and the identity theory of mind, and it remains one of the most influential frameworks for understanding the nature of mentality.

Background and Core Idea

Functionalism holds that what makes something a particular mental state is not what it is made of, but what it does. To be in pain, for example, is not to be in a specific brain state as such, nor merely to exhibit pain behaviour, but to occupy a state that is typically caused by bodily injury, interacts with beliefs and desires, and tends to result in avoidance behaviour and expressions of discomfort. The defining feature of the state is its causal role within the overall mental system.
This perspective places functionalism at an intermediate theoretical level between physical implementation and overt behaviour. While neuroscience investigates the physical mechanisms of the brain, and behaviourism focuses on observable stimuli and responses, functionalism concentrates on the organisation of internal states and processes that mediate between input and output. This level of description is often compared to software in relation to hardware: just as a computer program can be realised on different physical machines, mental states can, in principle, be realised in different physical systems.

Functionalism and Its Rivals

Functionalism developed largely in response to limitations in earlier theories of mind. Cartesian dualism posited two fundamentally different substances, mental and physical, which raised enduring problems about how these substances could interact. Behaviourism, particularly in its logical and psychological forms, attempted to define mental states purely in terms of behavioural dispositions, but struggled to account for the apparent internal complexity of mental life and for cases where mental states exist without corresponding behaviour.
The identity theory of mind proposed that each type of mental state is identical with a particular type of brain state. While this view preserved a commitment to physicalism, it faced difficulties in explaining how creatures with very different biological constitutions could nevertheless share similar mental states. Functionalism offered a solution by rejecting type-identities between mental states and physical states, and instead identifying mental states with functional roles that could, in principle, be realised in different physical media.

Multiple Realisability

A central concept in standard functionalist theories is multiple realisability. According to this idea, a single type of mental state can be realised by different kinds of physical systems, so long as those systems implement the appropriate functional organisation. The analogy often used is that of a valve: a valve can be made of metal, plastic, or other materials, yet still count as a valve provided it performs the function of regulating flow.
Applied to the mind, this means that a mental state such as pain could be realised in a human brain, an animal nervous system, or even a suitably designed artificial system. A silicon-based machine, for example, could in principle have mental states similar to those of human beings if its internal structure realised the same functional roles. Multiple realisability has been used to argue that psychology cannot be straightforwardly reduced to neuroscience, since the same psychological state may correspond to many different physical states.

Functional Specification Theories

Not all theories labelled functionalist accept multiple realisability. Functional Specification Theories (FSTs), most notably associated with David Kellogg Lewis and David Malet Armstrong, offer a more restrictive account. According to FSTs, a mental state is not the functional role itself, but the particular physical or neurological state that occupies that role in a given system.
On this view, to say that pain is defined by a certain functional role is to specify which physical state counts as pain in humans, namely whatever brain state actually realises that role. Mental states are therefore essentially tied to their physical realisers, and multiple realisability across radically different systems is rejected. An alien or artificial system might have a state that plays a similar functional role to human pain, but it would not be pain in the same sense.
Critics of FSTs argue that this consequence is counterintuitive, especially in hypothetical cases involving aliens or artificial intelligences that behave and reason in ways indistinguishable from humans. Defenders reply that similarity of function does not entail identity of mental state, and that the physical realisation is an essential component of mentality.

Machine-State Functionalism

One of the earliest and most influential forms of functionalism is machine-state functionalism, developed by Hilary Putnam in the 1960s. This version draws a close analogy between the mind and a computational machine, specifically the abstract model known as a Turing machine. A Turing machine is defined not by its material construction but by a set of states and transition rules that determine how it responds to inputs and produces outputs.
In a Turing machine, each state is characterised entirely by its relations to other states and to inputs and outputs. The internal material details are irrelevant. Putnam suggested that mental states could be understood in a similar way: each mental state is defined by its place in a system of causal relations, rather than by its physical composition. Since Turing machines are abstract entities, any system capable of passing through the relevant succession of states could, in principle, implement the same mental processes.
Putnam later became critical of machine-state functionalism, partly through thought experiments such as Twin Earth, which raised concerns about whether functional role alone could determine mental content. Nevertheless, machine functionalism played a crucial role in establishing functionalism as a serious alternative to both behaviourism and mind–brain identity theories.

Psychofunctionalism

A second major variety is psychofunctionalism, closely associated with philosophers and cognitive scientists such as Jerry Fodor and Zenon Pylyshyn. Psychofunctionalism rejects the idea that mental states can be adequately characterised using only commonsense or a priori functional descriptions. Instead, it holds that mental states should be identified with the internal states postulated by our best empirical psychological theories.
On this view, psychology is an autonomous and irreducibly complex science, much like biology. Mental terms such as belief, memory, or perception refer to states defined by their roles within sophisticated cognitive models, rather than by simple stimulus–response patterns. Psychofunctionalists argue that reducing these states to behavioural dispositions would be neither feasible nor desirable, as it would strip psychological explanations of their explanatory power.

Significance and Implications

Functionalism has had a profound influence on philosophy, psychology, and cognitive science. It provides a framework for understanding the mind that is compatible with scientific investigation while avoiding the limitations of strict reductionism. By emphasising organisation and function, it supports the idea that mental processes can be modelled computationally and studied independently of their specific physical realisations.
At the same time, functionalism has faced sustained criticism. Objections include concerns about subjective experience, or qualia, which some argue cannot be captured purely in functional terms. Others question whether functional roles are sufficient to fix mental content or whether they overlook the importance of biological embodiment.