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Since the dawn of what Norbert Elias called 'the process of civilisation' sight has held an undisputed place as the most estimable of the senses. Asked what he had been born for, one of the earliest philosophers, Anaxagoras, replied: 'For seeing.' Even Heracleitus, 'the obscure', is supposed according to Polybius to have asserted that 'eyes are surer witnesses than ears'. That cornerstone of Western philosophy, Aristotle's Metaphysics, opens with the assertion: 'Most of all, we esteem the sense of sight.' Hippocrates, the father of medicine, for all that he became a bookish authority for later generations, also pleaded for the evidence of things seen. Indeed, by the fifth century BC, the faculty of vision and the attributes of knowledge were already one: the Greek verb theorein means both to see and to know.

So when Greek ideas were rediscovered at the beginning of what we call the Renaissance it wasn't just those trouble-some antithetical terms from the pre-Socratic era, especially that conceptual pair light-dark, the former being 'noble' (like male, dry and right), the latter 'ignoble' (like female, wet and left), that were given a new lease of life. The mind's eye beheld the world afresh: knowledge was once more analogous to vision. The greatest figure of the Renaissance, Leonardo da Vinci, restricted his studies to the domain of what he called sperienza, by which he meant chiefly visual experience. Ignorance was ...