Functionalism

5. Responses to inverted spectra

There are various responses to the retinal operation thought experiment open to both ‘role’ and ‘realizer’ functionalists. Let us consider them in turn, starting with role functionalism.

(1) The most direct response open to role functionalism is simply to deny the intuition that Matthew will have different conscious experiences from normal people. That is, role functionalists can argue that if Matthew’s state A plays the same functional role as the state produced in normal people by red things, then it will feel the same, even if it is physiologically different. After all, they can point out, by hypothesis this state will make Matthew react in just the way that normal people react to red experiences, both in his behaviour and in forming further beliefs and desires. If this is so, they will ask, then what substance is there to the hypothesis that the experience is nevertheless consciously different?

However, this answer is less than wholly convincing. Maybe some qualitative states, like pain, can plausibly be argued to depend on nothing but functional role (if a state creates a pressing and intense desire to move some part of your body, does that not show it is a pain?). But other qualitative states, of which colour experiences are the paradigm, do not seem to be nearly so closely tied to functional role (there is no specific desire or behaviour which is typically prompted by an experience of green). So if we take some such state, like normal people’s experience of green, and imagine its functional role switched while its physiology stays the same, as in Matthew, then most philosophers have a strong intuition that will still feel the same, despite the fact this runs counter to role functionalism.

(2) Other role functionalists adopt a different tack. They simply admit that their theory of mind cannot deal with ‘qualia’ – that is, with the qualitative aspects of colour and similar conscious experiences. Instead they accept that these aspects are fixed by physical realization rather than functional role. Role functionalists of this stripe can continue to maintain a role functionalist account of non-qualitative states like belief and desire, and indeed of the non-qualitative aspects of qualitative states. But their role functionalism will be incomplete, in that they admit that qualia themselves are not fixed by their functional role. In particular, they concede that an individual who is functionally normal but physically abnormal, like Matthew, will not experience normal qualia.

(3) The ‘retinal operation’ thought experiment is much less of a problem for realizer functionalism than for role functionalism. After all, realizer functionalists take mental terms to refer to physical states from the start: they distinguish between human pains (one physiological state) and octopus pains (a different physiological state), even though both states play the pain role. Similarly, they can distinguish between Matthew’s experience of red, which is one physical state, and a normal person’s experience, which is a different physical state. True, both states play the same functional role. But nevertheless the realizer functionalist counts them as distinct mental states, and to this extent need not be disturbed by intuitions that they are also qualitatively distinct.

(4) Both the realizer functionalist of the last paragraph, and the incomplete role functionalist of the previous one, take qualia to be fixed by physical realization. Different physics, different qualia. However, some philosophers argue that this principle is excessively ‘chauvinist’, since it implies that beings who lack human physiology cannot share human experiences. Surely, more ‘liberal’ philosophers argue, we want to allow that dolphins, say, or the inhabitants of Proxima Centauri’s third planet, might be functionally organized in such a way as to experience pain and sadness, say, even if they have non-human physiologies.

David Lewis has devised an ingenious version of functionalism, which allows this kind of liberalism while still respecting the intuition that Matthew’s experience when he is faced with something green is like a normal person’s experience of red. Lewis argues that terms for mental states (‘belief’, ‘desire’, ‘pain’, ‘sadness’, ‘experience of green’ and so on) should be understood, in their application to any given being, to refer to the first-order state that realizes the relevant functional role in normal members of the species (or other group to which the being belongs). Matthew is a human being, so ‘experience of green’ as applied to him refers to the physical state that is produced by green things and plays the corresponding functional role in normal human beings. In Matthew, of course, this physical state (state A) plays the functional role that relates to red things. But classified mentally it is still an experience of green, because in normal people it plays the role appropriate to green things.

If we are talking about a Proxima Centaurian, by contrast, then ‘experience of green’ does not refer to the physical state produced by green things in humans, but rather to the physical state (if there is one) produced by green things in Proxima Centaurians. So it is possible for a Proxima Centaurian to have an experience of green, Lewis argues, as long as it is in the physical state that plays the relevant role in its normal conspecifics, even if that is different from the state A that plays this role in humans. (Note that there could also be an abnormal Proxima Centaurian in whom the experience of green does not play the green functional role, as long as it is the state that plays that functional role in normal Proxima Centaurians.)

So, according to Lewis, qualitative experiences vary with physical realizer states within a given group, but with functional role states across groups. His theory thus accommodates the intuition that Matthew is qualitatively different from normal humans, while avoiding the chauvinist implication that animals and extraterrestrials cannot share our experiences.

Lewis is normally classified as a realizer functionalist. But in one respect the mixed theory just outlined goes beyond realizer functionalism. A different physical state realizes experiences of green in humans and Proxima Centaurians. Yet Lewis counts them alike in respect of their qualitative nature. This makes him different from the realizer functionalists described in (3) above, who take the difference in physical realization to lead to a difference in qualitative feel. Still, this only shows that Lewis is not a straightforward realizer functionalist, not that he is wrong.

A more serious difficulty for Lewis’ theory is that it makes the qualitative classification of peoples’ mental states depend on which species or group we assign them to. Suppose a minority subspecies of humans evolves a different pain mechanism from other humans. And suppose Jane belongs to this subspecies. Qua member of the subspecies, Jane will be in pain when her mechanism is activated, for she will be in the state that plays the pain role in normal members of the subspecies. But qua human simpliciter, she will not be in pain, for she is not in the state that plays the pain role in normal humans. Lewis accepts that there will sometimes be no unique answer to the question of which group an individual belongs to, and consequently that it will be indeterminate what qualitative mental states it has. This is, to say the least, a surprising consequence of his theory.

(5) Some role functionalists argue that the way to deal with the inverted spectrum problem is to distinguish extra levels of functional organization. Standard discussions only consider two levels – ‘macroscopic’ functional role and physical realization. Because of this, standard discussions face a dilemma: they either tie qualia to ‘macroscopic’ functional role, and fail to accommodate the intuition that Matthew is qualitatively abnormal; or they tie qualia to physical realization, and end up ‘chauvinistically’ denying pains to Proxima Centaurians. Lewis’ mixed theory offers one way out. But another way out would be to uphold role functionalism, but identify some intermediate level of ‘micro-functional’ organization, which will distinguish Matthew from other humans, yet be common to humans and members of different species. By hypothesis, Matthew’s states coincide with normal human states in respect of such ‘macroscopic’ role features as which actions they give rise to, which beliefs and desires they prompt, and so on. But they need not coincide in more ‘microscopic’ respects such as the light reflectance profile being computed, the firing pattern of the relevant neural units, and so on. The important point is that these microscopic features are still functional, in that they can be realized in systems of different physical composition. So they could be shared by humans and physically different species, while at the same time serving to distinguish Matthew from normal humans.

Note that this ‘microfunctional’ solution is open to scientific functionalists, but not to common-sense functionalists. Assumptions about reflectance profiles and neuronal firing are not part of common-sense psychology. Common-sense functionalism is therefore forced to count Matthew as functionally normal, and so unable to explain his intuitive qualitative difference by reference to functional factors. Scientific functionalism is not so constrained, as there are many possible levels of functional organization in the brain which might distinguish Matthew from normal humans. The only question is whether this is an embarrassment of riches. For once we have opened up the possibility that qualitative features might be fixed by any level of functional organization, what could possibly decide which level does in fact fix it? It is no good appealing to the subject’s introspective reports, since they are part of macro-functional role, which is already agreed not to fix qualia (thus Matthew will describe state A as ‘seeing red’, yet nearly everybody agrees he is experiencing green). On the other hand, it seems highly unlikely that intuition can resolve the issue, for intuition is surely not fine-grained enough to decide such questions as whether somebody who is computing the normal reflectance profile for green things, but using an abnormal pattern of neuronal firing to do so, is experiencing green or not.