More sense from those who lack one

How do you know but ev'ry Bird that cuts the airy way,/Is an immense world of delight, clos'd by your senses five?" Bryan Magee, without being as poetical as Blake, ponders a similar question. Reality might be quite other than the way it manifests itself to us via our five senses, yet we cannot get outside our "epistemological cage". When religion or philosophy purport to do so they produce mere verbiage - you can only think on both sides of a limit if you can get beyond that limit, and to get beyond our sensory limits we would need a physical apparatus other than the one we have, or perhaps our own extended by an extra sense. But could we not, asks Magee, get an inkling of what that would be like by comparing our experience as fully sensed people with that of an intelligent person who had always lacked one of the senses? Getting to understand what it is like to have one less sense could illuminate what it would be like to have one more. On Blindness consists of Magee's correspondence with the late Martin Milligan, head of philosophy at Leeds, who counts as born blind, having been blind as far back as he could remember.

Unlike so many of the academic philosophers from whom he differentiates himself, Magee actually asks important questions. His aim is to increase our understanding of the human condition, and though Milligan died before addressing the particular questions their correspondence was meant to pursue, On Blindness amply fulfils this intention, even if not in the way intended. It is largely a dispute on the nature of knowledge, in which Milligan, determined to establish that the blind can know what it is to see, even though they cannot experience it, vehemently rejects Magee's claim that all knowledge is based on sensory experience. He invokes Wittgenstein's private language argument - that no one could confidently assume his or her understanding of what other people say they know if language consisted initially of naming, and then referring to, our individual experiences of external objects or of internal sensations. In order to learn the meanings of words, and share them, subjective experience must "drop out of consideration as irrelevant". The way Milligan writes is in fact a convincing substantiation of Wittgenstein's argument. He speaks of recognising the differences between "glances and stares, glimpses and full views, loving looks and freezing looks, quizzical looks and solemn looks". And he says he can grasp the idea of the visual and emotional effects of different light frequencies by drawing parallels between them and auditory clashes and consonances, and can acquire the same connotations for darkness as the seeing, even though he can never experience darkness precisely because, from the sighted viewpoint, he experiences nothing else.

Milligan's evocative account of appreciating the "brownness" of coffee through its taste, the colours of sounds, and the varying texture and brightness of these colours - soft, dark, mellow, garish, and so on - adds a further dimension to Wittgenstein's notion of language as a common pool. Although Magee does not draw these conclusions, the interchangeablity of sensory concepts highlighted by Milligan suggests that our epistemological cage is not as internally compartmentalised as we might assume: there is perhaps a cross-sense sense enabling sensory concepts to apply along a sliding sensory scale, equally to colour, texture, sound, smell, taste and temperature. But also beyond. A warm smile, a slippery customer, a dense argument - even when the adjectives in those phrases are applied in the literal contexts of temperature or texture, they have an inner lining of emotion and appraisal. Similarly they are able to apply to nonsensory qualities of personality or conversation because these are already charged with materiality. Metaphor works because of all-pervasive factors in human experience.

In a final letter to the reader Magee speculates whether, since Milligan was able to infer so much about the visual sense he lacked, five-sensed humans might not do something analogous with human experience as a whole. Yet surely, though in one way Milligan's being conversant with visual experience is reassuring, in another the very fact that he could easily "pass as" sighted underlines the paradox that experience in general can be communicated and compared only because it (or at least its specificity) cannot be. Rather than extending our sensory horizons, we are reminded of the seamless, but ineluctable net of language, and learn more of its extraordinary nature. And that itself is fascinating.

Jane O'Grady is an extramural philosophy teacher for Birkbeck College, London, and co-compiler with A. J. Ayer of A Dictionary of Philosophical Quotations.