Free Russell - A Very Short Indroduction by A. C. Grayling Page B

differences of colour and shape. Ordinary common beliefs, and belief in the existence of other minds, are still excluded.
    With this enriched basis of what he now calls ‘hard data’, Russell formulates the question to be answered thus; ‘can the existence of anything other than our own hard data be inferred?’ His approach is first to show how we can construct, as a hypothesis, a notion of space into which the facts of experience – both the subject’s own and those he learns by the testimony of others – can be placed. Then, to see whether we have reason for believing that this spatial world is real, Russell gives an argument for believing that other minds exist, because if one is indeed entitled to believe this, then one can rely on the testimony of others, which, jointly with one’s own experience, will give powerful support to the view that there is a spatial – that is, a real – world.
    This strategy is ingenious. In the paper ‘The Relation of Sense-Data to Physics’, written in early 1914, Russell adds to it an equally ingenious way of thinking about the relation of sense-experience to things. In PP he had said that we infer the existence of physical things from our sensedata; now he describes them as functions of sense-data, or as he sometimes puts it, ‘constructions’ out of sense-data. This employs the technique of logic in which one thing can be shown to be analysable into things of another kind. Russell describes as the ‘supreme maxim of scientific philosophising’ the principle that ‘wherever possible, logical constructions are to be substituted for inferred entities’. In accordance with this principle, physical objects are accordingly to be analysed as constructions out of sense-data; yet not out of actual or occurrent sense-data only, but out of ‘sensibilia’ also, by which is meant ‘appearances or, in Russell’s phrase, ‘how things appear’, irrespective of whether they constitute sense-data which are currently part of any perceiver’s experience. This is intended to explain what it is for an object to exist when not being perceived.
    An important aspect of this view is, Russell now holds, that sense-data and sensibilia are not private mental entities, but part of the actual subject-matter of physics. They are indeed ‘the ultimate constituents of the physical world’, because it is in terms of them that verification of common sense and physics ultimately depends. This is important because we usually think that sense-data are functions of physical objects, that is, exist and have their nature because physical objects cause them; but verification is only possible if matters are the other way round, with physical objects as functions of sense-data. This theory ‘constructs’ physical objects out of sensibilia; the existence of these latter therefore verifies the existence of the former.
Instead of developing this distinctive theory further, Russell abandoned it; in later work, particularly in The Analysis of Matter ( AMt ) in 1927 and Human Knowledge ( HK ) in 1948, he reverted to treating physical objects, and the space they occupy, as inferred from sense-experience. A number of considerations made him do this. One was his acceptance, driven by the sciences of physics and human physiology, of the standard view they offer that perception is caused by the action of the physical environment on our sense organs. ‘Whoever accepts the causal theory of perception’, he writes, ‘is compelled to conclude that percepts are in our heads, for they come at the end of a causal chain of physical events leading, spatially, from the object to the brain of the percipient’ ( AMt 32). He also, in The Analysis of Mind ( AMd ) in 1921 gave up talk of ‘sensedata’, and ceased to distinguish between the act of sensing and what is sensed. His reason for this relates to his theory of the mind, sketched later.
    Another major reason for Russell’s abandonment of the theory was the sheer