The Simple Objectivist view is that the objective essences of colours are manifest. The more common form of objectivism, however, is that colours have hidden objective essences. Advocates of this view have differed, though, on whether the hidden essence of colour is a microstructural feature, intrinsic to the coloured body or whether it is a light-related dispositional property, e.g., one connected with reflectance profiles. Thomas Reid was probably the earliest example of the first type and Frank Jackson (1996) the latest. Recent examples of the second type are D.M. Armstrong (1969), J. Westphal (1987) and D. Hilbert (1987).

The attempt to locate the essence of colour among the microstructural features of coloured bodies seems unpromising. One of the major problems is `the problem of multiple realisations'. Given the range of bodies that have colours _ surfaces, volumes, light-sources, illuminations, luminescent bodies, films, expanses _ the intrinsic physical features that provide the causes for the way colours appear show a bewildering variety. Even if we concentrate on the first type of colour, surface colour, : we find that there is a wide variety of underlying physical microstructures, responsible for objects' appearing blue, yellow, etc. The causes of the colours objects appear to have are many and varied. The same type of micro-structure consistently appears the same colour (within limits) under different conditions, but different microstructures may appear the same.

It would seem, therefore, that the most plausible candidates for objective essences are light-related dispositional properties, e.g., capacities to emit, reflect, absorb, transmit or scatter light to varying degrees. However, the problem of multiple realisations has merely been postponed for, depending upon the type of object in question, a different candidate for the objective essence has to be found. For physical surfaces, it would have to be related to the object's reflectance curve, e.g., the capacity to differentially reflect wavelengths from different regions of the incident illumination; in the case of volumes, it would be related to the object's transmittance; in the case of such objects as the sky, to the scatterance; in the case of aperture colour or film colour, it would be related to the pattern of light received at a particular place or at the source of the light (the reflecting source in the case of physical surfaces) and so on. Even in the case of ordinary objects, the colours may be caused in a variety of ways. The blue of a bird's coat may result from scatterance, the red in a different way.

Nevertheless, progress can be made if we concentrate on one type of colour, surface colour. The most plausible attempt is to try to identify surface colour with a light-modifying disposition, e.g., with a disposition to reflect (or absorb) certain proportions of standardised illumination, or, if one prefers, certain proportions of light of the wavelengths from the visible spectrum. Objects with neutral or achromatic colours are ones which reflect all wavelengths to (roughly) the same degree, with whites reflecting a higher percentage than greys and blacks. Objects with chromatic colours are those which differentially reflect or absorb light at different wavelengths. Accordingly surface colour would be identified with some feature of an object's spectral reflectance curve.

A special case of the problem of multiple realisations is posed by the occurrence of metamers. In the case of physical surfaces there are metamers, i.e objects with very different reflectance curves that have identical appearances of colour. The situation is far more pronounced in the case of film colours or aperture colours. Here there are innumerable different combinations of light that will give the same hue. It would seem that the property shared by physical objects with the same film colour is a disposition to incite the three light-sensitive cones in the retinae according to the same ratio: x: y: z.

The major problem, however, with any of the objectivist accounts of surface colour is that, given the way the natural or folk concept has been characterised, it is hard to see how there could be an objective essence with the right characteristics. For, given the natural or folk concept of colour, it seems that colour is a certain kind of property: a perceiver-independent, intrinsic property of objects, one that satisfies certain constraints. But it is hard to see how there could be any intrinsic, physical features that satisfy all the constraints. The most crucial requirement is that colours, as a group, have to stand together of in the right kind of relationships. There are no manifest, intrinsic features that satisfy this requirement. Nor does any set of microstructures stand the remotest chance of satisfying the appropriate constraints. In particular, there are no physical features, either of microstructure, or of the object's contribution to light, such that the right kind of internal relationships hold. [See Hardin (1988) and Maund (1995).] Neither can colour be a dispositional property, say spectral reflectance, or a disposition to produce physiological responses of a certain kind, since spectral reflectances don't fit together in the right kind of ways.

There are two kinds of response an objectivist might make. One would be to deny the account of the natural concept of colour as expounded here. Instead, it is held, it is part of the way colour terms operate in the language, that it is understood that colours may well turn out to be hidden essences. The other response involves not challenging the account of the natural concept but insisting instead that it needs to be revised or reconstructed for, say, scientific purposes. (As shown by the discussion in the next section of scientific examples of identification and reduction, these examples often involve revisions or reconstructions of previous concepts.) With this response the objectivist can concede that there are no colours in the way ordinarily conceived but hold that, nevertheless, there are colours as conceived in another manner, i.e., in an objectivist manner. After all, there are not atoms as Dalton conceived them, nor oxygen as Lavoisier conceived it, nor planets as pre-Aristotelians conceived them, but still there are atoms, oxygen and planets, all the same. In other words, the objectivist can propose a revisionary concept of colour as an objective property, e.g some microstructural property or a spectral reflectance or some other light-modifying feature.

The assessment of this proposal will depend on the nature of the revision recommended. It is one thing to propose the introduction of a new concept; it is another thing to propose it as a replacement for the existing concept in the spirit of eliminating it and other competing revisions. There is no reason in principle why we should not introduce a new, different concept of colour, `physical colour' which, we may assume, takes over the causal role specified in our characterisation of colour. Such a move is legitimate, but it leaves open the possibility that there is still a need for another concept, for the causal requirement was only one of the requirements for the original concept. If the new physical concept cannot service other legitimate requirements, then we need another concept to serve these purposes. One possibility is that two new concepts should emerge. As Ian Hacking has pointed out, it is plausible that the original concept of "acid" later split into two new concepts, each perfectly legitimate. Another example is the replacement of Newtonian mass by the two new concepts of mass in relativity theory. Where previously it had been assumed that there was a single essence, it is now the case that there are two essences.

Assessing the merits of any revisionary proposal will depend on examining the reasons for modifying the original concept, and on whether there is any available rival. Before considering the objectivist's revisionary proposal, let us consider the other response the objectivist can make: to reject the account offered above of the folk or natural concept. There are two forms this response can take, a simple form and a more sophisticated one. In the simple version, the way colours appear is used as a criterion for detecting the presence of the hidden essence, but it is not essential to what it is for something to be coloured. After all, gold is acknowledged as having both a hidden essence and an appearance: the real essence of gold, its atomic number, plays a causal role in producing a certain golden appearance, one that is used by language-users to identify, loosely, pieces of gold. In like manner, it is argued, colours have hidden essences which play a causal role in their having the appearances they do.

As an account of how colour terms operate, this view is implausible. It has the consequence that the appearance is not essential to colour; that if objects were to cease to have their distinctive appearances, while retaining their reflectance-profiles, then our colour vocabulary would largely remain untouched. Our summary of the colour-principles, however, revealed that all of these principles either directly involve colour appearances or it was the case that the way they worked was through colour appearances. Without appearances, colours would not be of any interest whatever. Just as wines would cease to have interest, even to dedicated wine-growers, were they to lose their distinctive tastes, so too would colours were they to lose their appearances. There are two possibilities. One is that through genetic change, humans became incapable of seeing objects except in terms of shades of grey. So no object has the distinctive colour appearances. The second situation is the same as the first, except that 40 years later, technologists have devised spectacles (or implants) that allow people once again to see objects as coloured. However they cannot match appearances with reflectances. Tomatoes have become blue, skies appear red except at sunsets when they are appear blue ( or sometimes greenish-blue). It seems implausible that in such circumstances colour vocabulary would go with the light-dependent properties rather than with the appearance dispositional properties.

It needs to be remembered that the situation as far as colours are concerned is very different from that for gold or aluminium. As Putnam points out, though most of us identify gold through its appearance, the appearance does not constitute (part of) the essence for gold. The reason we do so is that the appearance is a trivial criterion. It is easy to imagine circumstances in which gold would lose its lustre and in any case we distinguish fools' gold from real gold. The situation is very different for colours. Unlike gold the appearance associated with colours is crucial. The important point, as far as colours are concerned, is that coloured objects have characteristic appearances and that those appearances are of great interest to us. It is because we have that interest that we need a concept of dispositional colour - the power to appear in characteristic ways. It is because of the way colours appear that they are important to us both biologically and socially. It is because coloured bodies appear that way (i.e., the way they do) that colours perform their various functions. To tie fool's yellow with appearance and real yellow with some microstructural property seems absurd. Of course, if we distinguish between say physical colour and psychological colour, then we could imagine circumstances in which two objects which had the same psychological yellow colour were ones in which one object had real physical yellow, and the other fool's physical yellow.

There is, however, a different way of construing the natural concept, which disarms the force of some of these considerations. It is to hold that the disposition to appear is not part of what colours are essentially, but it is part of the (folk) conceptualisation of a certain perceiver-independent property, which is colour. The folk, it is held, use the way colours appear in order to characterise a certain property, but the way objects appear is essential to the characterisation, not the property. Here we are depending on a distinction between a property and the mode of presentation (or Fregean sense) through which the property is presented (or is thought about). Accordingly, appearances would be tied to the concept of colour without being part of the property of colour. It would then be open to us to identify the property through some scientific means.

In so far as we have a dispute about how the folk concept should be construed, it seems to me that this objectivist account is simply wrong and that Hacker, Maund, Price, Hering and Boynton are right: that colours are manifest, intrinsic features of objects. Consequently they are not dispositional properties having to do with capacities to emit, reflect, absorb, transmit or otherwise modify light. Moreover there are things we can say about the kind of properties colours are (given this concept): they are properties that fit together in the characteristic 3-dimensional structured arrays. Accordingly the folk concept is different from what the objectivist requires.

Having said that, it may not matter a great deal. For even if the objectivist's proposal gets the folk concept wrong, we can assess it as offering a revisionary proposal, i.e., as a reconstruction of the folk concept. The spirit of the proposal is that the objectivist concept is all that any self-respecting folk would want. In any case, if it is true, as defenders of the illusion theory hold, that there are no colours, as the folk conceive them, it can hardly be surprising that a revisionary proposal is called for. There is, however, a competing revisionary proposal: that colours are mind-dependent dispositional properties, e.g., powers to appear in certain ways to perceivers of a certain kind. To assess the merits of these rivals and indeed to see whether they need to be seen as rivals, we should consider some parallel examples in science.