page 3 of Russell, Experience, and the Roots of Science

Russell was not, as noted, concerned to address scepticism. His tack was to say that although sceptical arguments are strictly speaking irrefutable, there is nevertheless 'not the slightest reason' to suppose them true (PP p 17). Instead he assembles persuasive considerations in support of the view that having sense-data provides access to reasonable knowledge of things in space. First, we can take it that our immediate sensory experiences have a 'primitive certainty'. We recognise that when we register sense-data which we naturally regard as associated with, say, a table, we have not said everything there is to be said about the table. We think, for example that the table continues to exist when we are not perceiving it, and that the same table is publicly available to more than one perceiver at a time. This makes it clear that a table is something over and above the sense-data that appear to any given subject of experience. But if there were no table existing independently of us in space we should have to formulate a complicated hypothesis about there being as many different seeming-tables as there are perceivers, and explain why nevertheless all the perceivers talk as if they were perceiving the same object.

But note that on the sceptical view, as Russell points out, we ought not even to think that there are other perceivers either, for if we cannot refute scepticism about objects, we are as badly placed to refute scepticism about other minds.

Russell short-circuits the difficulty by accepting a version of the argument to the best explanation. It is simpler and more powerful, he argues, to adopt the hypothesis that, first, there are physical objects existing independently of our sensory experience, and, secondly, that they cause our perceptions and therefore 'correspond' to them in a reliable way. Following Hume, Russell regards belief in this hypothesis as 'instinctive'.

To this, he argues, we can add another kind of knowledge, namely, a priori knowledge of the truths of logic and mathematics. Such knowledge is independent of experience, and depends only on the self-evidence of the truths known. When perceptual knowledge and a priori knowledge are conjoined they enable us to acquire general knowledge of the world beyond immediate experience, for the first kind of knowledge gives us empirical data and the second permits us to draw inferences from it.

These two kinds of knowledge can each be further divided into subkinds, described by Russell as immediate and derivative knowledge respectively. He gives the name 'acquaintance' to immediate knowledge of things. The objects of acquaintance include particulars, that is, individual sense-data (and perhaps ourselves), and universals. Derivative knowledge of things Russell calls 'knowledge by description', which is general knowledge of facts made possible by combination of and inference from what we are acquainted with.

Immediate knowledge of truths Russell calls 'intuitive knowledge', and he describes the truths so known as self-evident. These are propositions which are just 'luminously evident, and not capable of being deduced from anything more evident'. For example, we just see that '1 + 1 = 2' is true. Among the items of intuitive knowledge are reports of immediate experience; if I simply state what sense-data I am now aware of, I cannot (barring trivial slips of the tongue) be wrong.

Derivative knowledge of truths consists of whatever can be inferred from self-evident truths by self-evident principles of deduction.

Russell concedes that despite the appearance of rigour introduced by the availability of a priori knowledge, we have to accept that ordinary general knowledge is only as good as its foundation in the 'best explanation' justification and the instincts which render it plausible. Ordinary knowledge amounts at best therefore to 'more or less probable opinion'. But when we note that probable opinions form a coherent and mutually supportive system – the more coherent and stable the system, the greater the probability of the opinions forming it – we see why we are entitled to be confident in them.

An important feature of Russell's theory concerns space, and particularly the distinction between the all-embracing public space assumed by science, and the private spaces in which the sense-data of individual perceivers exist. Private space is built out of the various visual, tactual and other experiences which a perceiver co-ordinates into a framework with himself at the centre. But because we do not have acquaintance with the public space of science, its existence and nature is a matter of inference.

IV

Thus Russell's first version of a theory of knowledge, and because its chief outlines are found in PP it is the one most familiarly associated with his name. But he was by no means content with the expression of it in PP, which after all was a popular book and did not essay a rigorous exposition of its theses. The technical papers, TK and OKEW which followed were his considered versions of these same questions, and mark an advance over this first sketch. One difference between the theories of PP and OKEW is that Russell had come to see that the experiencing subject's basis for knowledge – the sense-data that appear to him alone, and his intuitive knowledge of the laws of logic – is insufficient as a starting point. He accordingly placed greater weight on an experiencer's memories, and his grasp of spatial and temporal relations holding among the elements of occurrent experience. The subject is also empowered to compare data, for example as to differences of colour and shape. Ordinary common beliefs, and belief in the existence of other minds, are still excluded.

This appeal to an enriched conception of cognitive capacities required at the foundations of knowledge is almost invariably made by empiricist epistemologists – consider Locke and Ayer also – when the thin beams of sensory experience and inference are found, as they invariably are, to be insufficient to bear the weight of knowledge.

With this enriched basis of what he now called 'hard data' Russell reformulated the question to be answered thus: 'can the existence of anything other than our own hard data be inferred?' His approach was first to show how we can construct, as an hypothesis, a notion of space into which the facts of experience – both the subject's own and those he learns by others' testimony – can be placed. Then, to see whether we have reason for believing that the 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 underwrite the view that there is a spatial (a real) world.

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