There are two basic philosophical problems about colour. The first concerns the nature of colour itself. That is, what sort of property is it? When I say of the shirt that I am wearing that it is red, what sort of fact about the shirt am I describing? The second problem concerns the nature of colour experience. When I look at the red shirt I have a visual experience with a certain qualitative character – a ‘reddish’ one. Thus colour seems in some sense to be a property of my sensory experience, as well as a property of my shirt. What sort of mental property is it?
Obviously, the two problems are intimately related. In particular, there is a great deal of controversy over the following question: if we call the first sort of property ‘objective colour’ and the second ‘subjective colour’, which of the two, objective or subjective colour, is basic? Or do they both have an independent ontological status?
Most philosophers adhere to the doctrine of physicalism, the view that all objects and events are ultimately constituted by the fundamental physical particles, properties and relations described in physical theory. The phenomena of both objective and subjective colour present problems for physicalism. With respect to objective colour, it is difficult to find any natural physical candidate with which to identify it. Our visual system responds in a similar manner to surfaces that vary along a wide range of physical parameters, even with respect to the reflection of light waves. Yet what could be more obvious than the fact that objects are coloured?
In the case of subjective colour, the principal topic of this entry, there is an even deeper puzzle. It is natural to think of the reddishness of a visual experience – its qualitative character – as an intrinsic and categorical property of the experience. Intrinsic properties are distinguished from relational properties in that an object’s possession of the former does not depend on its relation to, or even the existence of, other objects, whereas its possession of the latter does. Categorical properties are distinguished from dispositional ones. A dispositional property is one that an object has by virtue of its tendency to behave in certain ways, or cause certain effects, in particular circumstances. So being brittle is dispositional in that it involves being liable to break under slight pressure, whereas being six feet tall, say, is categorical. If subjective colour is intrinsic and categorical, then it would seem to be a neural property of a brain state. But what sort of neural property could explain the reddishness of an experience? Furthermore, reduction of subjective colour to a neural property would rule out even the possibility that forms of life with different physiological structures, or intelligent robots, could have experiences of the same qualitative type as our experiences of red. While some philosophers endorse this consequence, many find it quite implausible.
Neural properties seem best suited to explain how certain functions are carried out, and therefore it might seem better to identify subjective colour with the property of playing a certain functional role within the entire cognitive system realized by the brain. This allows the possibility that structures physically different from human brains could support colour experiences of the same type as our own. However, various puzzles undermine the plausibility of this claim. For instance, it seems possible that two people could agree in all their judgements of relative similarity and yet one sees green where the other sees red. If this ‘inverted spectrum’ case is a genuine logical possibility, as many philosophers advocate, then it appears that subjective colour must not be a matter of functional role, but rather an intrinsic property of experience.
Another possibility is that qualitative character is just a matter of features the visual system, in the case of colour, is representing objects in the visual field to have. Reddish experiences are just visual representations of red. But this view too has problems with spectrum-inversion scenarios, and also entails some counterintuitive consequences concerning our knowledge of our own qualitative states.
Faced with the dilemmas posed by subjective colour for physicalist doctrine, some philosophers opt for eliminativism, the doctrine that subjective colour is not a genuine, or real, phenomenon after all. On this view the source of the puzzle is a conceptual confusion; a tendency to extend our judgements concerning objective colour, what appear to be intrinsic and categorical properties of the surfaces of physical objects, onto the properties of our mental states. Once we see that nothing qualitative is happening ‘inside’, we will understand why we cannot locate any state or property of the brain with which to identify subjective colour.
The controversy over the nature of subjective colour is part of a wider debate about the subjective aspect of conscious experience more generally. How does the qualitative character of experience – what it is like to see, hear and smell – fit into a physicalist scientific framework? At present all of the options just presented have their adherents, and no general consensus exists.
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