Functionalism

4. Inverted spectra

Now to criticisms of functionalism: one family of objections focuses on functionalism’s ability to deal with conscious, qualitative states – states that it is ‘like something to have’, in Thomas Nagel’s 1974 phrase. These states include sensory experiences, pains, itches, and emotions, and so on, but arguably do not include such propositional attitudes as belief and desire (see Qualia; Propositional attitudes.)

Let us start with the ‘inverted spectrum’ argument against functionalism. Suppose that baby Matthew has an operation performed on his retina at birth which switches the ‘red’ and ‘green’ messages from his retina to his visual cortex: the central physiological state produced in Matthew by red things is thus the state normally produced by green things in other people, and vice versa. From then on Matthew is brought up normally, learning how to discriminate between red and green things, to call them ‘red’ and ‘green’ respectively, and so on. Consider now the state (let us call it A) produced in Matthew by red things. It seems likely, given Matthew’s normal upbringing, that A will play the same causal role as the physiologically different state which arises when other people are presented with red things. Now let us ask: what kind of conscious experience will Matthew have when he is in state A? Despite A’s sharing a causal role with the state normally produced by red things, it seems intuitively plausible that A will be consciously like the state normally produced by green things. After all, when Matthew has A, his brain is physiologically just like a normal brain presented with a green thing.

But this now presents a prima facie problem for functionalism. For if the functional role of A classifies it with the state normally produced by red things, but it feels like the state which is normally produced by green things, then functionalism has failed to capture the conscious aspect of this state.

Note how this thought experiment differs from the simpler, traditional ‘inverted spectrum’, which postulates an individual – Millie, say – who is normal in all physical respects but still has her colour experiences ‘inverted’. The physical state which gives normal people the conscious experience of red gives her the conscious experience of green. Millie is incompatible with the general physicalist assumption that there cannot be mental differences without physical differences of some sort. The ‘retinal operation’ version of the inverted spectrum thought experiment, by contrast, does not require us to deny this general physicalist assumption. There are physical abnormalities in Matthew to account for his abnormal colour experiences. The point of the retinal operation thought experiment is rather that Matthew’s abnormality is only at the physical level, and not at the functional level. Because of this, the retinal operation thought experiment poses a problem specifically for functionalism, which is committed to explaining mental differences in terms of functional differences, but not necessarily for other versions of physicalism.