Harvard University,  FAS

Philosophy 156

Philosophy of Mind

Asst. Prof James Pryor
Dept. of Philosophy

The Inverted Spectrum (as an Argument against Functionalism)

Experiences have two different kinds of features.

It is controversial how these two kinds of features are related to each other. Some philosophers believe that they are in fact the same features: that two experiences differ qualitatively iff they differ representationally. Other philosophers believe that these two kinds of features come apart: that two experiences might be the same in the one respect, while differing in the other respect. We will return to this debate later. For now, we will leave it an open question how these two kinds features are related.

Philosophers sometimes use the word "qualia" as a name for our experiences' qualitative features. However, this word gets used in different ways, so we have to be careful:

  1. Some philosophers use the word "qualia" in a neutral way, as a name for our experiences' qualitative features, without begging any questions about how those qualitative features should be understood. All theorists can accept the existence of "qualia" in this sense. They just disagree about what gives our experiences the "qualia" they have.

  2. Other philosophers use the word "qualia" in a more loaded way, as a name for some specifically intrinsic, non-relational features of our experiences, which give those experiences their qualitative character. If "qualia" of this sort exist, they are a problem for the functionalist. The functionalist wants to define all mental states in terms of the causal relations they stand in. But "qualia" of this sort would be mental states which have to be understood in terms of their intrinsic nature, rather than in terms of the relations they stand in.

  3. Other philosophers use the word "qualia" in a different loaded way, as a name for some specifically non-physical features of our experiences, which give those experiences their qualitative character. If "qualia" of this sort exist, they are a problem for the physicalist.

When a philosopher talks about "qualia," you'll have to determine for yourself which of these meanings he is giving the term. In our discussion, I'll stick to the expression "qualitative features," and leave it an open question whether these qualitative features have to be understood in intrinsic or non-physical terms. It might turn out that the functionalist and the physicalist are unable to give a satisfactory account of what gives our experiences their qualitative features. But if this is true, it will take philosophical argument to show it. It won't be part of the definition of "qualitative features."

The Inverted Spectrum Thought-Experiments

Inverted spectrum thought-experiments can be used to raise a number of problems. For now, we're going to concentrate on the use of these thought-experiments in arguments against functionalism. To do this, these thought-experiments have to convince us that two things are true in the situations they describe:

  1. two subjects have qualitatively different experiences, when confronted by the same stimuli

  2. despite that fact, however, the subjects are functionally equivalent: their qualitatively different experiences stand in the same causal relations to other mental states and to input and output

If a thought-experiment succeeds in doing this, then it will have shown that there can be a mental difference between two subjects without there being any functional difference. So there are facts about the subjects' mental lives which are not captured by the functionalist story.

We can picture a normal person's mental states working as follows:

That is, when a normal person is confronted with a red object, that causes him to have experiences with certain distinctive qualitative features (for lack of a better name, we will call these features "reddish") and which represent the world as containing something red. In the normal course of events, these experiences will cause the person to form the belief that there is something red, and this belief will affect the person's behavior in certain ways (for instance, it will affect his verbal behavior: he will call the object he sees "red").

Suppose that Fred lives a normal life up to the age of 20. But then at age 20, we perform an operation on his optic nerve which causes green things (like grass) to look to him the way that red things (like ripe tomatoes) used to look, and vice versa. Immediately after the operation, Fred will be in this situation:

Inversion Stage 1

That is, green objects will look red to him, and he may initially be fooled into believing that those objects really are red, and calling them "red." At this point, Fred is subject to a systematic perceptual illusion. But eventually Fred will learn of his mistake, and he will suppress his initial inclination to take his experiences at face value. We might represent that as follows (with the link from experience to belief "broken"):

So far, this story does not raise any problems for the functionalist. For at this point, Fred is functionally very different than he was before the operation. If you showed pre-op Fred a ripe tomato and asked "What color is this?" he would answer "red." If you show a ripe tomato to Fred at this point, and ask its color, he would answer "I don't know. It looks green, but I can't trust my experiences anymore." Fred behaves differently in response to the same stimuli.

Suppose, though, that Fred gets accustomed to his new experiences. Now, when you show him a ripe tomato, although it looks to him the way that green things used to look, he figures out that the tomato must therefore be red, because that's how red things look now. This adjustment may come to be second nature for Fred. When that happens, Fred will be in a situation like this one:

Inversion Stage 2

Comment: Many philosophers argue that at this stage, Fred is rather in the following situation:

Inversion Stage 2 (Alternate)

We will compare and contrast these two models of Fred's situation later, when we investigate the relation between representational and the qualitative features of experience.


At this point, the functionalist should start to get worried. For Fred's greenish experiences are beginning to take on the functional role formerly occupied by his reddish experiences (and vice versa).

The proponent of the inverted spectrum thought-experiment will continue to push the story, trying to portray a situation in which the functional roles of Fred's reddish and greenish experiences are wholly swapped. In the end, Fred will be behaviorally and functionally indiscernible from a normal perceiver. Yet when he sees red things, he has greenish experiences, not reddish experiences. This difference in his experience does not manifest itself in his behavior or in his use of color words.

If this is really possible, then the reddish and greenish qualitative features of Fred's experiences cannot be given a functional definition.

Comment: A different way to tell inverted spectrum stories involves two subjects, whose experiences are inverted relative to each other, rather than a single subject who undergoes an operation (which inverts his experiences relative to the way they used to be). For instance, we might take identical twins and implant spectrum-inverting lenses in the eyes of the one twin, while it's still a fetus. When the twins grow up, we show them both ripe tomatoes, and teach them to call things which look that way "red." (Even though this is a different way of looking for each twin!) And so on. Block sketches such a case in "Troubles with Functionalism":
It makes sense, or seems to make sense, to suppose that objects we both call green look to me the way objects we both call red look to you. It seems that we could be functionally equivalent even though the sensation that fire hydrants evoke in you is qualitatively the same as the sensation grass evokes in me. Imagine an inverting lens which when placed in the eye of a subject results in exclamations like "Red things now look the way green things used to look, and vice versa." Imagine further, a pair of identical twins one of whom has the lenses inserted at birth. The twins grow up normally, and at age 21 are functionally equivalent. This situation offers at least some evidence that each's spectrum is inverted relative to the other's. (p. 221)
There are some differences between these inverted spectrum cases and the case of Fred. The basic structure of the cases is very similar, however. And if either case is genuinely possible, then the qualitative features of our experiences must not be captured by the functionalist story.

Assessing the Thought-Experiments

In thinking about the inverted spectrum thought-experiments, it is important to keep several issues disentangled. The thought-experiments can be viewed as raising some epistemological problems:
  1. If inverted spectrums are really possible, then how can we know what each other's experiences are like? And how can we tell whose experiences represent the true colors of objects? (Do objects have true colors?)

These are real and serious questions. However, they are not our topic here. Instead, we are interested in the metaphysical problems that the thought-experiments raise. Our problem here is:

  1. Is it possible for two subjects to have qualitatively different experiences, when the experiences are functionally equivalent (i.e., when the experiences stand in the same causal relations to inputs, outputs, and other mental states)?

The proponent of the thought-experiments says that this is possible, and hence, qualitative features of experience cannot be given a functional definition.

The functionalist will try to resist. He will concede that situations like Inversion Stage 1 (and perhaps even Inversion Stage 2) are possible. But he will deny that it is possible for experiences that are qualitatively different from your own to play all the same functional roles that your experiences play. If the experiences are qualitatively different, then that must make some sort of functional difference.

A different way to respond to the inverted spectrum thought-experiments is to concede that qualitative features of experience cannot be given a purely functional definition. This still leaves it open that those qualitative features might be given a physicalist definition. Perhaps the qualitative features of our experiences can be explained in terms of the experiences' neurophysiological features. That is, one might be an Identity Theorist about these features.

A popular combination of views these days is to be a functionalist about propositional attitudes (and also about the representational features of experience), while being an Identity Theorist about the qualitative features of experience.

[Phil 156] [James Pryor] [Philosophy Dept.]

Created by: James Pryor
Last Modified: Mon, Jul 17, 2000 6:58 PM