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

that case, the event is both mental and material at once’ ( Portraits from Memory (1958), 152). This, for consistency, is what he should have argued in AMd itself, where only sensations have this character. But this view in turn generates another problem, which is that it comes into unstable tension with a view to which Russell returned after AMd , namely, that the causes of percepts are inferred from the occurrence of the percepts themselves. As noted earlier, Russell wavered between treating physical things as logical constructions of sensibilia and as entities inferred as the causes of perception; he held this latter view in PP and returned to it after AMd . But on the face of it, one is going to need a delicate connection between one’s metaphysics and one’s epistemology in order to hold both that minds and things are of one stuff, and that things are the unknown external inferred causes of what happens in minds. So those parts of the legacy of AMd which remain in his later thinking raise considerable difficulties for his later views about matter.
    Realism and perception
    One of the chief reasons for Russell’s reversion to a realistic, inferential view about physical things was the difficulty inherent in the notion of unsensed sense-data or, in the later terminology, percepts. As noted above, the idea had been to replace inferred entities with logically constructed ones, an application of the analytical technique. lf physical things can be logically constructed out of actual and possible sensedata, then two desiderata have been realized at once: the theory is empirically based, and inferred entities have been shaved away by Ockham’s razor. But it is obvious, and the point has already been made, that the idea of unsensed sense-data (or unperceived percepts) is, if not indeed contradictory, at least problematic. It makes sense – although, without a careful gloss, it is metaphysically questionable – to talk of the existence of possibilities of sensation; but to talk of the existence of possible sensations arguably does not (note Russell’s definition of sensibilia as entities having the ‘same metaphysical and physical status as sense-data without necessarily being data to any mind’). If the choice lay between inferred material particulars and non-actual perceptions existing unperceived, it would seem best to plump for the former. In effect, this is what Russell himself came to think; and unsensed sensations went out of the window. But he did not return to the cruder form of inferential realism held in PP ; something more ingenious, but no more successful, was up his sleeve, as explained shortly.
    Another reason for Russell’s reversion to realism was his recognition that the notion of causality is problematic for phenomenalism. Things in the world seem to affect one another causally in ways that are difficult to account for properly by mere reports of sense-experiences. Moreover, a causal theory of perception is a natural and powerful way of explaining how experience itself arises. In Russell’s mature philosophy of science, contained in AMt and Human Knowledge (1948), he did not opt for a Lockean view which says that our percepts resemble their causal origins – the so-called ‘picture-original’ theory – because we cannot be directly acquainted with things, and therefore cannot expect to know their qualities and relations. Rather, he now argued, changes in the world and our perceptions are correlated, or co-vary, at least for orders of things in the world that our perceptual apparatus is competent to register (we do not, for example, perceive electrons swarming in the table, so there is no associated covariation of world and perception at that level). The correspondence between percepts and things is one of structure at the appropriate level: ‘Whatever we infer from perceptions it is only structure that we can validly infer; and structure is what can be expressed by mathematical logic’ ( AMt