Colour and qualia

1. Physicalism and objective colour

The question ‘What sort of property is the redness of the shirt?’ is sensible only against a background of assumptions regarding what counts as an informative answer. Otherwise, the following naïve response seems appropriate. Colour is one of the features physical objects have, and red is a type of colour. What else is there to say? What makes the question substantive, however, is the background assumption of the doctrine of physicalism.

Physicalism is a doctrine accepted by a wide variety of philosophers, despite wide disagreement about its detailed characterization. There are two basic tenets: (1) that there is a privileged level of basic physical properties, its precise inventory being the business of physical science to determine; and (2) that all causal transactions in nature occur by virtue of mechanisms that are ultimately realized by the properties of this basic class. This doctrine is sometimes expressed by the slogan, ‘no change without a (basic) physical change’ (see Materialism in the philosophy of mind; Supervenience of the mental).

Acceptance of physicalism generates a problem about colour in the following way. The colour of my shirt – its being red – has causal influence; most notably, on the visual experiences of sentient beings. By the second principle of physicalism there must be physical mechanisms that mediate this influence. So how do these mechanisms relate to the colour?

There seem to be four possible answers. First, colour is itself a basic physical property, akin to mass, charge, charm and the like. Second, colour is reducible to other physical properties, in something like the way that heat is reducible to mean kinetic energy or water is reducible to H₂O (see Reductionism in the philosophy of mind). Third, there is no natural physical property with which to identify colour: colours are mind-dependent properties in the sense that they can only be characterized by reference to their effects on human visual systems. Finally, colours are not real properties of objects at all: in a sense they are an illusion (see Colour, theories of; Property theory).

The first alternative is clearly a non-starter, since colour is not a basic physical property. There is controversy about the second. To explain the controversy, we must review some basic facts about colour vision. What we normally think of as perceived colour is a combination of three properties: hue, brightness and saturation. Hue (essentially the shade of colour itself) is determined by the wavelength composition of the light hitting the eye; brightness is determined by the relative intensity of the light; and saturation is a matter of how much hue relative to white light there is in the stimulus.

Now, it might be thought that a specific colour could be identified with light of a specific wavelength, intensity and saturation. Then the colour of a physical object could be identified with a tendency to reflect just that composition of light. However, it turns out that bundles of light with quite different distributions of wavelengths can produce the same effect on the visual system and therefore are perceived as the same colour. (Such distributions are called ‘metamers’.) Also, there is the phenomenon of colour constancy. As illumination changes, say from bright outdoors to indoors, or noon to late afternoon, the composition of the light reflected from my red shirt changes. Yet it continues to look red throughout. One standard explanation of colour constancy is that the visual system takes changes of illumination into account by comparing the light reflected from many objects at once, thus using the contrast as a major determinant of perceived colour. But this makes the identification of colour with any property of the light reflected from an object quite complicated, if not hopeless.

In response to this problem, David Hilbert (1987) proposes that we identify colour with the complex, but quite objective, property of ‘spectral reflectance’. An object’s spectral reflectance is a function that takes specific distributions of light contained in illuminants as input and specific distributions of light reflected as output. He argues that it is precisely this property that is preserved in cases of colour constancy. What we perceive when we perceive the shirt as red throughout changes of lighting conditions is its surface spectral reflectance. There are various problems with this view, the most notable being the problem of metamers. As mentioned above, two very different light distributions, even against the same background illumination, can yield the same perceived colour. Yet, in Hilbert’s view, since the two objects reflecting these two different distributions have different spectral reflectances, they must count as differently coloured. This seems quite counterintuitive, especially if the background illumination in question is broad daylight. (Hardin (1988) dismisses the spectral reflectance view largely for this reason. Hilbert (1987) responds at length to the objection, and the interested reader should consult his discussion.)

In view of the difficulties with the second alternative, it might be thought that colour is best thought of as a disposition on the part of objects to cause certain experiences in us. So what makes the shirt red is its disposition, however grounded in its physical properties, to cause a normal human observer, under normal conditions, to have a reddish visual experience. (The historical roots of this position can be found in Locke’s discussion of primary and secondary qualities; see Primary–secondary distinction; Locke, J. §4.) Note that on the dispositional view, colour is a real, but mind-dependent property of objects. That is, objects are really coloured, though their being so is dependent on properties of us.

Hardin (1988) disputes the dispositional view on the grounds that there is no principled analysis of the phrases ‘normal conditions’ and ‘normal human observer’. Thus there is no unequivocal way to attribute colours to objects, since as we change observer and conditions we change the object’s colour. On his view the appropriate locus of colour attribution is experience: there are chromatic experiences, but no coloured objects. In our terms, there is only subjective colour, no objective colour.