Black from White

When I was around the age of seven, my parents had close friends who lived a few doors away from Camberwell School of Art, where I think they had studied some years earlier. They had a daughter, Esmie, who was three years my junior and I remember she once said something that struck me as obviously wrong - she claimed that white was not a colour. I tried to disabuse her of this misconception only to be gently corrected by our dads. I don't recall their explanation or if they even gave one but ever since I have always harboured a distrust of white.

Strictly speaking, Esmie was right that white is not a colour, at least not a chromatic colour. Look in the Chambers English Dictionary though, and the first entry you will find states: "White /wīt or hwīt/ adj of the colour of snow, the colour that reflects the maximum and absorbs the minimum of light rays." As we will see, getting to grips with white, turns out to be a little more difficult than might at first seem the case. White, as it turns out, is a bit of a grey area.

When the dads stepped into my dispute with young Esmie over the subtleties of colour definitions, perhaps they responded with something like: "White is actually a combination of all the colours." That, and the fact that I was obviously in the minority, might have shut me up for a while but a quick visit to a set of poster paints would soon have demonstrated the inadequacy of this explanation. No amount of deft colour mixing—at least of the kind that I was familiar with—would ever have generated white. Why otherwise was there a block of white in all the sets of paint I had ever owned if not for the fact that it is impossible to concoct such a non-colour from a combination of all the colours? Were my parents trying to taunt me with my obvious inability to blend colours in precisely the right formulation? Was the art of colour combination simply too sophisticated for a mere child equipped with a set of cheap paints? When in later childhood I discovered that watercolour sets standardly include no white, my worst suspicions seemed confirmed. Quite how the four-year-old Esmie could have understood such complexities escapes me. Perhaps her collection of felt-tip pens revealed a deeper secret.

Of course the answer to this conundrum lies in the distinction between subtractive and additive techniques of colour-mixing. Dyes or pigments are the components of the subtractive system because, when mixed, they subtract from white or—strictly speaking—they absorb more wavelengths of light. The additive system on the other hand mixes coloured light - the equal combination of red blue and green (yes green) combining to form white (for humans that is).

So far so good, whiteness consists of the combination of equal quantities of visible spectra. But the problems really only start here because although we have established that white is not a chromatic colour, we haven't yet established much else besides. Indeed, if it weren't for the fact that we can easily select instances of white in the world then the challenge of exemplifying the concept would be almost impossible. One possible strategy might be to say that white is the opposite of black. This is helpful but we're not out of the woods yet, far from it, in fact we're heading straight into a thicket and by the time we're through to the other side I hope to have convinced you that, in certain circumstances and without changing its chemical composition or intensity at all, white can become a perfectly serviceable simulation of black.

All we need for this thought experiment is a dimmish room, a white screen and a projector. So long as the environment isn't too bright then we should be able to see any image cast into the screen by the projector. First imagine that the projector is off. Every normally sighted human capable of expressing a rational opinion about what they see, would describe the screen in this state as white. For the sake of accuracy let's also check the intensity of light reflected from the screen to determine that it is currently reflecting an exposure value (EV) of 5 which is perfectly good for reading etc. but isn't too bright for our purposes.

Now let's turn the projector on and cast a blank rectangle of light into the screen. The physical properties of the screen haven't changed so it seems likely that everyone would still agree that the screen remains white—though the illuminated parts are now obviously brighter than the unlit parts. If we take another light-measurement of these dark portions of the screen we will find that they remain at an EV of 5 (i.e. what everyone agreed was white).

Now, let's project a simple greyscale image into the screen, an image of a room with black and white floor tiles and let's ask the audience whether the tiles look black and white. Once again everyone will agree - the tiles in the image look black and white. We can also take another light-measurement of the black tiles and once again we will find that it remains at 5, or even possibly slightly brighter.

So, what's going on? How can an objectively white thing seem so uncontentiously black? Philosophers and psychologists, like well-meaning dads, are tempted to jump in at this point to inform us that the phenomenon is the result of an illusion in which we misjudge, misread or miscalculate the white as black. Essentially they take the view that illusion is dependent upon a failure of cognition of some kind. Failure does indeed play an instrumental role in the explanation of what is going on, but the failure is definitely not one of reason (judgement), literacy (reading) or computation (calculation), nor in fact is it a failure of perception: it is a failure of sensory discrimination.

So, let's set the question of illusion aside here and start instead with a simple explanation of sensory discrimination. All organisms are engaged with the world by means of their senses. If senses are triggered, organisms respond. Brains then, are part of sophisticated systems of responsiveness in which organisms possess both genetically acquired and learned responses to the things they encounter. Some stimuli will inevitably be more important than others and it will therefore be in each organism's interests to respond advantageously to relevant stimuli and to be unresponsive to irrelevant stimuli. Sensory discrimination then, is a vital capacity that enables organisms to respond in different ways to different stimuli.

Equipped with this understanding of sensory discrimination, we can return to the projected image scenario and discover that we need not assume any failures of reasoning at all. All we need to assume is that the depicted black tiles trigger many of the same sensory responses as would actual black tiles seen from the depicted viewpoint. On a perceptual level we would have no difficulty in differentiating between the simulated black and an actual instance of black. And if we were practiced in skills of rational judgement there is no reason why these could not enable us to recognise the differences between simulation and reality. Nonetheless it is important to repeat that nothing hangs on these skills. Nor would any improvement in our skills of judgement make the slightest difference to our susceptibility to discrimination failure, to the seeing—in this case— of white as black.