page 4 of Russell, Experience, and the Roots of Science

This strategy is ingenious. In 'The Relation of Sense-Data to Physics' Russell adds an equally ingenious way of thinking about the relation of sense-experience to its objects. In PP he had said that we infer the existence of physical things from sense-data; now he described them as functions of or 'constructions' out of sense-data. This employs the technique of logic in which a thing of one (more complex) kind can be shown to be analysable into things of another (simpler) kind. Russell was here relying on what he called the 'supreme maxim of scientific philosophising', namely the principle that 'wherever possible, logical constructions are to be substituted for inferred entities.' Concordantly with this principle, physical objects are to be analysed as constructions out of sense-data – but not out of actual or occurrent sense-data only, but out of possible sense-data too. For actual and possible sense-data Russell coined the term 'sensibilia' by which is meant 'appearances' or, in Russell's phrase, 'how things appear', irrespective of whether they constitute sense-data 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, Russell now held, is that 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.

V

Such was the epistemology Russell developed in the period to 1914. Instead of developing this distinctive theory further, Russell abandoned it. In later work, particularly AMt and HK, 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 of the standard view offered by physics and physiology that perception is caused by the action of the environment on our sensory surfaces. 'Whoever accepts the causal theory of perception,' he wrote (AMt p 32), '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'. In AMi he gave up talk of 'sense-data', and ceased to distinguish between the act of sensing and what is sensed. His reason for this relates to his acceptance – long in coming, for he had repeatedly resisted it in print – of James's 'neutral monism'.

Another reason for Russell's abandonment of the sensibilia theory was the sheer complexity and, as he came to see it, implausibility of the views he tried to formulate about private and public spaces, the relations between them, and the way sensibilia are supposed to occupy them. He makes passing mention of this cluster of problems in MPD, before there reporting, as his main reason for abandoning the attempt to construct 'matter out of experienced data alone,' that it 'is an impossible programme ... physical objects cannot be interpreted as structures composed of elements actually experienced' (MPD p 79). This last remark is not strictly consistent with Russell's stated view in the original texts that sensibilia are not, and do not have to be, actually sensed; MPD gives a much more phenomenalistic gloss to the theory than it originally possessed. But it touches upon a serious problem with the theory: which is that it is at least problematic to speak of an 'unsensed sense-datum' which does not even require – as its very name seems per contra to demand – an intrinsic connection to perception.

In these early endeavours Russell gave only passing attention to other important questions in epistemology which he later, by contrast, came to emphasise. They concern the kind of reasoning traditionally supposed to be the mainstay of science, namely, non-demonstrative inference. It was some years before Russell returned to consider these questions: the main discussion he gives is to be found in HK, but promissory notes are issued in AMt and IMT.

VI

Acceptance of James's 'neutral monism' was an important turning point. Summarily stated, James's theory is that the world ultimately consists neither of mental stuff, as idealists hold, nor material stuff, as materialists hold, nor of both in problematic relation, as dualists hold, but of a neutral stuff from which the appearance of both mind and matter is formed. By Russell's own account, he was converted to this theory soon after finishing LLA. He had written about James's views in 1914, and rejected them; in LLA itself he was more sympathetic, though still undecided; but finally in a paper entitled 'On Propositions' (1919) he embraced the theory, and used it as a basis for AMi.

The question that came to seem key to Russell is whether consciousness is the essence of the mental, given that, in line with traditional views, consciousness is itself taken to be essentially intentional. In light of Russell's difficulties with the multiple relation theory of judgment it is pointful to remember its partial ancestry in Meinong's view that the intentional relation has at least the three elements of act, content and object. In accepting neutral monism Russell was abandoning the irreducible assumptions of any such view. First, he says, there is no such thing as the 'act'. The occurrence of the content of a thought is the occurrence of the thought, and there is neither empirical evidence nor theoretical need for an 'act' in addition. Russell's diagnosis of why anyone might think otherwise is that we say, 'I think so-and-so', which suggests that thinking is an act performed by a subject. But he rejects this, for reasons similar to those advanced by Hume, who held that the notion of the self is a fiction, and that we are empirically licensed to say no more, on occasions of specifying them, than that there are bundles of thoughts.

Secondly, Russell criticises the relation of content and object. Meinong and others had taken it that the relation is one of direct reference, but in Russell's view it is more complicated and derivative, consisting largely of beliefs about a variety of more and less indirect connections among contents, between contents and objects, and among objects. Add to this the fact that, in imagination and non-standard experiences like hallucination, one can have thoughts without objects, and one sees that the content-object relation involves many difficulties – not least, Russell says, in giving rise to the dispute between idealists who think that content is more significant than objects, and realists who think objects are more significant than content. (Russell's use of these labels, although standard, is misleading: we should for accuracy substitute the label 'anti-realist' for 'idealist' here; this is because whereas, at bottom, realism and anti-realism are indeed differing theses about the relation of contents to objects, and thus are epistemological theses, idealism is a metaphysical thesis about the nature of the world, namely, that it is ultimately mental in character. This point is frequently missed in philosophical debate, so Russell is in good company. ) All these difficulties can be avoided, Russell claims, if we adopt a version of neutral monism.

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