Even for those generally sympathetic to functionalism, there is one category of mental states that seems particularly resistant to functional characterization. Functionalist theories of all varieties — whether analytic or empirical— attempt to characterize mental states exclusively in relational, specifically causal, terms. A common and persistent objection from the reactionaries, however, is that no such characterizations can capture the qualitative character, or “qualia”, of experiential states such as perceptions, emotions, and bodily sensations, or ontological subjectivity (Searle) of consciousness, since they would leave out certain of their essential properties, namely, “what it's like” (Nagel, 1975) to have them. This paper will present the most serious worries about the ability of functionalist theories to give an adequate characterization of these states.
Ned Block (1980) notes that there are two famous objections to functionalist theories of qualia: the Inverted Spectrum and the Absent Qualia Hypothesis. These are less in the form of argumentation as they are examples and counter-examples which tend to encourage intuitive responses in the counter-example form. The first move in the Inverted Spectrum objection consists in claiming that you might see red when I see green and vice-versa; likewise for the other colors so that our color experiences are phenomenally inverted. This does not suffice to create trouble for the functionalist yet. For you and I are surely representationally different here: for example, you have a visual experience that represents red when I have one that represents green. And that representational difference brings with it a difference in our patterns of causal interactions with external things (and thereby a functional difference). While phenomenally symmetrical yet completely inverted, we both use the same phrases when talking about our phenomenal experiences.
Shakespeare has expounded on this problem when he wrote, “A rose by any other name will smell just as sweet.” This reply can be handled by the advocate of inverted qualia, however, by switching to a case in which we both have visual experiences with the same representational contents on the same occasions while still differing phenomenally. The rose, then, does not smell so sweet. Whether such cases are really metaphysically possible, if it even matters, is open to dispute, however. Still, those authors who are representationalists about qualia would deny their possibility. Indeed, it is not even clear that such cases are conceptually possible. But leaving this to one side, it is consistent with functionalist thought that have to be some salient fine-grained functional differences between us for there to be alleged “phenomenal” differences, notwithstanding the fact about our having different identities.
Consider a computational example. For any two numerical inputs, M and N, a given computer always produces as outputs the product of M and N. There is a second computer that does exactly the same thing. In this way, they are functionally identical. Does it follow that they are running exactly the same program? Of course, not! There are all sorts of programs that will multiply together two numbers. These programs can differ dramatically. At one gross level the machines are functionally identical, but at local levels the machines can be functionally different.
In the case of you and me, then, I would claim that even if we are functionally identical at a coarse level - we both call red things ‘red’, we both believe that those things are red on the basis of our experiences, we both are caused to undergo such experiences by viewing red things, etc. - there are necessarily fine-grained differences in our internal functional organization. And that is why our experiences are phenomenally different.