The issue of 'qualia' is a complex and wide-ranging one in contemporary philosophy of mind. The concept makes its appearence in debates about the nature of consciousness, about functionalism, about physicalism. But there is no general agreement about whether qualia even exist; nor about what the implications are if they exist; nor what it is for them to exist. And in different debates, the concept recieves slightly different interpretations. All of which makes it very confusing for the beginner.
The idea of 'qualia' starts with the idea of 'phenomenal consciousness'. Consciousness, espeically the sort of consciousness involved in perception, sensation, and emotion, has a 'feel' to it, what Tye calls 'some distinctive experiential quality'. The phrase often used to try to capture this experiential quality is 'what it is like'. There is something it is like to taste beer, to see a red rose, to feel sad. ('What it is like' here isn't meant to be comparative, it is meant to pick out how the experience is for the subject. When we make comparisons between experiences, e.g. 'Seeing a red rose is like seeing a ripe tomato', we do so in virtue of what it is like to see a red rose in the sense meant here.) We can call the properties of an experience, which gives it its distinctive experiential quality, 'phenomenal properties'.
Phenomenal Properties and Qualia
Some people think qualia are just phenomenal propertiesl But this isn't accurate. Philosophers all agree that there are phenomenal properties; what they disagree about is whether the concept of qualia is the best explanation for these properties. In the debates, qualia are usually understood as intrinsic and non-intentional properties. Philosophers who believe there are qualia argue that phenomenal properties are best understood as intrinsic, non-intentional properties of experience. The two terms are quite difficult to understand.
An intrinsic property is one that its possessor (in this case, the experience) has in and of its own, not in virtue of its relations to anything else. Think of the smell of coffee. It is the smell 'of coffee' because of its relation to the substance of coffee. That it is 'of coffee', is not an intrinsic property. But how that smell smells is an intrinsic property (people who believe in qualia argue), because it would be that smell even if it wasn't caused by coggee (a rose by any other name would smell as sweet). Intrinsic properties of experience, then, also relate to the identity of the experience. On this account, pain wouldn't be pain if it didn't feel painful
Intentional properties are properties of a mental state that enables it to represent what it does. Many philosophers believe that intentional properties are based on causation. The smell of coffee wouldn't be of coffee if it wasn't reliably caused by coffee and not by other things. Intentional properties, then, are relational rather than intrinsic. They depend on the way the mental state 'hooks up' to the world and other mental states. Qualia, because they are intrinsic properties, are non- intentional properties.
Philosophers who argue for qualia, argue that phenomenal properties are qualia. It is in virtue of qualia that experience has the distinctive quality it does, qualia determines 'what it is like' to have a particular experience.
Qualia and Functionalism.
The possibility of qualia poses a strong objection to functionalism, because if qualia exists, functionalism can't be a complete account of mental states. According to functionalism, mental states just are states that fulfil a particular causal functional role, i.e. they have certain causal relations to typical sorts of experience, behaviour, and to other mental states. If phenomenal properties, those properties that are the basis of phenomenal consciousness, are qualia, then they don't fulfil any casual functional roles; so functionalism can't explain phenomenal consciousness. There are two kinds of examples used to show this
The Chinese 'brain'. Suppose, as functionalism does, that the mind just is the working brain. Suppose the population of China was fitted with radios which were connected up in just the same way that the neurons in the brain are connected up, and messages passed between them in the same way as between neurons. According to functionalism, this should create a mind; but it is very difficult to believe that there would be a 'Chinese consciousness'.
This is a version of Locke's thought experiment of the inverted spectrum. Suppose that you and I are looking at ripe tomatoes and fresh grass. We both say that the tomatoes are red, the grass is green. But the particular way tomatoes seem to me is the way grass looks to you, and vice-versa. We could be 'set up' to respond to questions about their colours in just the same way, and yet have different colour experiences.
One standard functionalist reply has been to say in these examples, the brain and the Chinese population, or me and you, are not, in fact, functionally identical. There are going to be small, but very important, difference. This argument is unresolved, but it is a question of whether (phenomenal) consciousness can be reduced to a functional analysis.
Qualia and Representation.
Whether functionalism is true or not, we may still ask whether qualia (non-intentional, intrinsic phenomenal properties) exist. To try and show that they do, Shoemaker has argued that it is possible to distinguish the qualitative aspect of an experience from its representational aspect. One way of doing this is to find an example of two experiences that have different phenomenal aspects, but identical intentional content. One of his strongest arguments relates, once more, to colour experience.
Colour, as a secondary property, must be defined in relation to subjects. Different physical properties can produce the same colour experience in us. Our experiences of colour have similarities and difference, and on the basis of these, we divide up the colour spectrum. Shoemaker argues that 'what similarity and difference relations we perceive in the world is a function of what relations of phenomenal similarity and difference relations hold among our experiences'. So we think ripe tomatoes are more similar in colour to cricket balls than to grass. We make this comparison 'in the world' on the basis of comparisons between what our experiences are like.
To show that this is not about content, imagine two people, Bill and Ben, looking at three samples of colour, A, B and C. Bill says B is more similar to A than to C; Ben disagrees, and says the B is more similar to C than to A. Given that they are looking at the same objects, so they are not seeing different colours (objectively speaking), the best explanation for this is that for Bill, the quality of his experiences of A and B are more similar than the quality of his experience of B and C; and vice-versa for Ben. Because Bill goes on to say that B is more similar to A than to C (not the experience of B is more similar to A than to C), and Ben disagrees, they do represent A, B and C differently. But this difference in content is a result of the difference in the qualia of their experiences. Qualia, Shoemaker says, are 'non-intentional features of experiences that somehow underlie their intentional features'.
This last point, that the similarity is represented as between the objects, gives philosophers that want to resist qualia hope, because it makes the connection between phenomenal properties and representation so close, that perhaps they cannot come apart.
Consider that qualities 'of experience' don't seem to attached to the experience at all, but to the object of experience. The smell of coffee is a property of coffee, the way a rose looks is a property of the rose; not the experience of these things (how can an experience smell like anything?). So it is more intuitive to say that the qualitative aspects of experience are a function of its content, what it represents, rather than attached to the state itself. If you've ever been confused as to whether 'qualia' were meant to refer to properties of the object (such as colour) or to properties of experience, this is why! When trying to say what an experience is like, we end up describing what is an experience of; 'the description of what is apparent to us is not independent of our appreciation of what experience is like, and... for some aspects of our experience, it is difficult to conceive of how they could be independent of how things appear to us'
The ideas of 'appearance' or 'how things seem' are representational notion, and so they must involve intentional context. As the quote from Martin shows, we can't understand phenomenal properties except in terms of intentional content. Doesn't this mean that they are the same thing? There are no qualia, because phenomenal properties are intentional properties. Shoemaker wants to reject this final move. Yes, we normally talk about phenomenal properties, including similarities and differences, in terms of 'what it is like', which is to talk in terms of representation. But this doesn't show how this kind of representation is possible, what its nature is. His argument from similarity and difference tries to show that the representation, the way things seem, is a result of, and to be explained by, qualia.