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Colours as Observational Properties

The second main claim made by the naïve realist is that colours are distinct from the physical properties of objects.

In saying that colours are distinct from the physical properties of objects, the naïve realist is not necessarily saying that are ‘perfectly simple’ properties whose nature cannot be described further; indeed, on the face of it this is inconsistent with the claim, outlined in yesterday’s post, that colours are mind-independent properties. (It is partly for this reason I prefer to call the position ‘naïve realism’ rather than ‘primitivism’.)

Rather, to say that colours are distinct from the physical properties of objects is to say that they cannot be reductively identified with properties that can only be described using vocabulary from the physical sciences. So, for instance, colours are not dispositions to reflect light in different proportions across the electromagnetic spectrum (‘surface reflectance profiles’), or microphysical properties of objects, as they are according to common contemporary forms of colour physicalism.

I spend three chapters of A Naïve Realist Theory of Colour defending the claim that colours are distinct from the physical properties of objects. One line of argument for this claim is grounded in the idea that our colour concepts are concepts of observational properties.

A property is an observational property when it is not possible for something to look to normal perceivers in normal conditions to have the property, but not in fact have it. The property of being an apple, for example, is not observational in this sense, because whether something is an apple depends upon properties that cannot be perceived. It is possible for something to look like an apple to normal perceivers in normal conditions, but not be an apple: for instance, if it is a perfect plastic replica of an apple. The same is arguably not true of properties like colour or shape. If something looks yellow, or looks spherical, to normal perceivers in normal conditions, then it tempting to say that it must be yellow or spherical. It isn’t clear that we can make sense of the idea of ‘fool’s colours’ or ‘fool’s shapes’: things that share the appearance of colours or shapes, but which aren’t really colours or shapes because they differ in their non-perceptible properties.

One way of presenting the general challenge to the colour physicalist is as follows: to square the claim that our colour concepts are concepts of observational properties with the claim that colours can be reductively identified with properties with a complex physical essence.

Consider, by way of illustration, a form of colour physicalism that identifies colours with types of ‘surface reflectance profile’. Most physical objects reflect light in different proportions right across the electromagnetic spectrum, and what determines the colour they will appear is the proportion of light that they reflect at different parts of the electromagnetic spectrum that is visible to humans, between roughly 400 and 700 nanometres. This is called their ‘surface reflectance profile’.

Colours cannot be identified with individual surface reflectance profiles, unless there are many more colours than we normally assume there to be. Objects that differ in their surface reflectance profile—often quite radically—can nevertheless appear identical in colour (at least under certain conditions). This is a phenomenon known as ‘metamerism’. Our reaction—or more specifically, lack of reaction—to the discovery that objects that differ physically can be identical in colour reflects, I suggest, the fact that our colour concepts are concepts of observational properties. Even after we have discovered that objects that appear identical in colour can differ physically, we nevertheless continue to treat them as identical. In this respect, our judgments about colours do not display a deference to science in the way that judgments about whether, say, whales are fish do.

Contemporary physicalists like Alex Byrne and David Hilbert accommodate the phenomenon of metamerism by identifying colours, not with individual surface reflectance profiles, but with types of surface reflectance profile that all appear identical in colour. But it seems possible to imagine that there could be objects that differed still more radically in their physical properties – for instance, perhaps they acted directly on the visual cortex – but which still appeared to be coloured. My sense is that we would treat these objects as coloured, too. The reason for this, I suggest, is that we ordinarily take colours to be observational properties that form a largely autonomous domain. Whether or not a colour ascription is true depends on the way an object appears to normal perceivers in normal conditions, not on its unobservable physical properties.

In the book, I develop this argument via a variation on Kripke’s modal argument against physicalist theories of mental states like pain. I also provide responses to two lines of objection to the claim that colours are distinct from the physical properties of objects: that colours understood in this way can play no role in causing colour experiences, and that there are no properties of physical objects that instantiate the appropriate structural properties of the colours (for instance, properties which stand in the right relations of similarity). These are interesting and important issues. In my final two posts, however, I want to step back from debates about the nature of colour and try to give a sense of some of the more general issues that thinking about colour can help to illuminate: questions about the nature and possibility of philosophical inquiry and the problem of consciousness.

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