  One first gives the notion of a normal human percipient with respect to colour for which there are objective tests in terms of ability to make discriminations with respect to colour. This can be done without circularity. Thus ‘discriminate with respect to colour’ is a more primitive notion than is that of colour. (Compare the way that in set theory ‘equinumerous’ is antecedent to ‘number’.) Then Smart elucidated the notion of colour in terms of the discriminations with respect to colour of normal human percipients in normal conditions (say cloudy Scottish daylight). This account of colour may be disjunctive and idiosyncratic. (Maxwell's equations might be of interest to Alpha Centaurians but hardly our colour concepts.) Anthropocentric and disjunctive they may be, but objective none the less. David R. Hilbert (1987) identifies colours with reflectances, thus reducing the idiosyncrasy and disjunctiveness. A few epicycles are easily added to deal with radiated light, the colours of rainbows or the sun at sunset and the colours due to diffraction from feathers. John Locke was on the right track in making the secondary qualities objective as powers in the object, but erred in making these powers to be powers to produce ideas in the mind rather than to make behavioural discriminations. (Also Smart would say that if powers are dispositions we should treat the secondary qualities as the categorical bases of these powers, e.g. in the case of colours properties of the surfaces of objects.) Locke's view suggested that the ideas have mysterious qualia observed on the screen of an internal mental theatre. However to do Locke justice he does not talk in effect of ‘red ideas’ but of ‘ideas of red’. Philosophers who elucidate ‘is red’ in terms of ‘looks red’ have the matter the wrong way round (Smart 1995).