Philosophers will divide over the question whether tastes, colours, sounds, smells and so on are by nature physical or phenomenal. If these so-called "secondary qualities" are physical, then it makes sense to think of one smell covering up another, which is still there and reappears when the smell covering it up is removed. Similarly, if you think of colour as a physical entity, or you thinkbof it as rather like paint , you can think of one "colour" covering another up, so that the top layer of "colour" could be peeled off to reveal the older underlying colour. But if you think of the qualities as inherently perceptual, then one colour or sound or smell is not covered up by another; it is replaced by it. As Plato puts it in the Phaedo , 'in such a situation it either withdraws or ceases to exist.' My own preference is for a theory in which the "secondary qualities" like tastes are not inherently physical. It makes no sense to think of a physical blue, say, that lacks the quality blue . But...

The positions and numbers of branches and leaves of trees and plants are governed by Fibonacci series: 1,1,2,3,5,8.13 . . . . , so there is order in their arrangement. Whether this pattern exists seems to have little or nothing to do with how used to it we are.

The fact is that the correspondence between colour and frequency is rough and approximate. To some "colours" (and what does this mean?) there corresponds no wavelength, or no single wavelength, of monochromatic light. Examples are the browns, the appropriately named "non-spectral" purples, and white. (Black is also an example!) The colours form a circle (roughly) or a three-dimensional solid of which the circle is a cross-section at middle brightness in fixed illumination. This is colour space; and the frequency scale does not really model its overal complexity, except around the one corner at the edge: the spectrum. This is related to the fact that there are three types of cone which respond to coloured light, not four. There is no photoreceptor which peaks in the yellow, so when we see yellow it is the "red" and "green" cones that are being stimulated. Scientists tend to draw the conclusion that "colour is a sensation", as Maxwell put it. But this can't be right, as I see it, for the most part for the...