Bare Functionalism Versus Digital Computational Functionalism

In philosophy of mind, people sometimes say things like the following: If functionalism is true about consciousness, then digital computers could be conscious; all it requires is that they instantiate the right programs. And yes, most functionalists about consciousness do think that. But the claim about digital computer programs does not straightforwardly follow from functionalism alone. Today I want to clarify the issue by distinguishing three forms of functionalism: bare functionalism, computational functionalism, and digital computational functionalism.

Bare Functionalism

According to bare functionalism -- that is functionalism, without further commitments -- mental states are functional states. They are determined wholly by their causal relations to stimuli, behavior, and other mental states. Whatever plays the causal role of some mental state M is mental state M. Whatever plays the causal role of pain, for example, being apt to be caused by tissue damage and tissue stress and being apt to cause writhing, groaning, complaint, avoidance, calls to the doctor, and angry vows of vengeance (to simplify somewhat) is pain -- whether it is brain state 1117A in humans, pod state 24uw in Martians, or computational state 0110100110 in a robot. It's also generally assumed that mental state terms are in principle eliminable en masse by Ramsification. That is, they are placeholders for whatever Xs, Ys, and Zs are caused by the specified stimuli and cause the specified behavior and have the right causal relations to other Xs, Ys, and Zs.

Thus, if two systems behave exactly alike given the same patterns of environmental stimuli and do so by going through the same types of abstractly specifiable internal state transitions, they are necessarily mentally identical, whether they are made of silicon, carbon, or plaster of paris. Of course some materials -- such as plaster of paris -- probably can't support the causal complexity of humanlike behavior, no matter how cleverly arranged. So that puts a practical limit on the feasible materials out of which a humanlike mind could be constructed. But the only constraint on the substrate or material underlying mentality is that it supports the relevant causal relations.

Of course, no two different materials will respond in exactly the same way to every perturbation. Functionalist substrate flexibility thus requires constraints on what count as relevantly different stimuli and relevantly different behaviors. Functionalists tend not to commit to specifics, but hand-waving generalizations convey the spirit: If we built a robot -- like one of Asimov's robots or Data from Star Trek -- who responded in broadly humanlike ways to broadly humanlike stimuli, and did so by means of internal mechanisms sufficiently like ours at an abstract causal level, the robot would have mentality like ours, whatever its underlying material constitution.

How similar must the internal mechanisms be? A single giant search tree or lookup table is probably too different. But suppose the subsystems segment the visual world into discrete objects, and store long-term memories, and store lexical items flexibly combinable via generative grammar, and monitor its bodily condition, goals, and mental states, and so on, for all of our most important cognitive functions abstractly described (or less question-beggingly, they do the abstract causal equivalent of all of these things). That's probably close enough, even if the details differ.

Computational Functionalism

Computational functionalism is stronger -- that is, riskier and more committal. It holds that functional computational structure alone suffices for consciousness. Bare functionalism needn't say this. Bare functionalism can require that stimuli, behavior, and internal causation be specified noncomputationally. Two important differences follow.

First, anti-computationalists, such as Searle, observe that a computational model of a hurricane gets no one wet and a computational model of an oven cooks no turkey. Unlike the computational functionalist, the bare functionalist can join Searle in insisting on wet turkey: To fully match the relevant input-output relations, the system much have certain concrete effects.

It might help to remember that functionalism is essentially a complexification of dispositional or causal approaches to properties. Just as we can define a poison as something that (simplistically) tends to sicken or kill when ingested, a functionalist can define a painful experience as something that tends to produce certain concrete behaviors, such as (simplistically) screaming, protest, and avoidance. If there's no tendency to concretely sicken, there's no poison; if there's no tendency to concretely scream, protest, and avoid, there's no experience of pain.

The computational functionalist rejects this commitment to specific, concrete inputs and outputs. As long as the right computational relations are instantiated, between some set of inputs and some set of outputs, I1...Ii and O1...Oj, abstractly described, mentality is present. This is because exactly the same computations have been executed, whatever the concrete Is and Os are.

Asimov's robots can throw water balloons and cook turkeys, and a functionalist who is not a computational functionalist can insist that this matters.

Second, although the bare functionalist holds that mental states depend on causal relations, unless pancomputationalism is true, not every causal relation is computational. On Piccinini's account of computation, for example, computation only occurs when "medium independent vehicles" -- physical variables defined solely in terms of their degrees of freedom (e.g., 0 vs 1) rather than their specific physical composition -- are manipulated according to rules. The bare functionalist can insist that some noncomputational causal relations matter to mentality.

Digital Computational Functionalism

Digital computational functionalism is stronger still. It further commits to treating the relevant computation digitally. Standard theories of computation, such as Turing's, are digital, and the best-known computers are digital computers. But other forms of computation are sometimes described and implemented, including analog computation and nondigital forms of computation inspired by, and perhaps instantiated in, the human brain.

Why This Matters

Keeping these distinctions clearly in mind can help us appreciate that functionalism is a larger tent than sometimes assumed. Functionalists don't need to think that digital computers could achieve consciousness. Functionalists can insist on genuine embodiment. Functionalists needn't insist that only abstractly specified causal relations matter. They do have to insist that what matters internally, among the Ramsifiable mental state relations, are the abstractly specified causal relations. But they can simultaneously insist on particular types of concrete inputs and outputs.

One might think this is an inelegant or flawed position, not the most natural development of functionalism; but that requires argument.

[COBOL Rube Goldberg by Philip Manker: image source]