THE PHILOSOPHY OF PAIN: THE SENSES
Aristotle said that were five senses: sight, sound, smell, taste and the body sense. By this he meant more than the obvious fact that there were five sets of organs responsible for the five senses. Each sense was a unique category in itself that could not be imitated by any mixture of the other four senses. It is true that the blind poet Milton wrote of 'the scarlet sound of the trumpets', but that refers to association 'synaesthesia' not to imitation. With human obsession with subdivision, these huge categories of sensation could not be left long to stand by themselves. Colour was itself subdivided into its primaries in such a way that mixtures of the three primaries could generate all the colours.
For skin senses, it was decided by introspection that there seemed to be four or six primary sensations: touch, warm, cold, pain, tickle and itch. As with colour, it was proposed that all other sensations were composed of mixtures of primaries so that, for example, wet was a mixture of touch and cold. Loud alarm bells should sound in anyone's brain on reading a list like the one above. It mixes up classes of words but invites you to treat them as the same class. Touch, warm and cold describe the stimulus; pain, tickle and itch describe the mental response, the sensation. In nineteenth-century terms, the sensation seemed so closely linked to the stimulus that it did not matter much which class of words were used, for light pressure seemed to equal touch and heavy pressure to the point of damage seemed to equal pain. We must therefore be very cautious and use one set of words for a stimulus event and another set for a perceived sensory event.
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