page 5 of Russell, Experience, and the Roots of Science

In James's view the single kind of 'primal stuff', as he called it, is 'pure experience'. Knowing is a relation into which different portions of primal stuff can enter; the relation itself is as much part of pure experience as its relata.

Russell could not go along with quite all of this. He thought that James's use of the phrase 'pure experience' showed a lingering influence of idealism, and rejected it; he preferred the use made by others of the term 'neutral-stuff', a nomenclatural move of importance because whatever the primal stuff is, it has to be able – when differently arranged – to give rise to what could not appropriately be called 'experience', for example stars and stones. But even with this modified view Russell only partially agreed. He thought that is right to reject the idea of consciousness as an entity, and that it is partly but not wholly right to consider both mind and matter as composed of neutral-stuff which in isolation is neither; especially in regard to sensations – an important point for Russell, with his overriding objective of marrying sense to physics. But he insisted that certain things belong only to the mental world (images and feelings) and others only to the physical world (everything which cannot be described as experience). What distinguishes them is the kind of causality that governs them; there are two different kinds of causal law, one applicable only to psychological phenomena, the other only to physical phenomena. Hume's law of association exemplifies the first kind, the law of gravity the second. Sensation obeys both kinds, and is therefore truly neutral.

Adopting this version of neutral monism obliged Russell to abandon some of his earlier views. One important change was abandonment of 'sense-data'. He did this because sense-data are objects of mental acts, which he now rejected; therefore, since there can be no question of a relation between non-existent acts and supposed objects of those acts, there can be no such objects either. And because there is no distinction between sensation and sense-data – that is, because we now understand that the sensation we have in seeing, for example, a colour-patch just is the colour-patch itself – we need only one term here, for which Russell adopts the name 'percept'.

Before accepting neutral monism Russell had objected to it on a number of grounds, one being that it could not properly account for belief. And as noted, even when he adopted the theory it was in a qualified form; mind and matter overlap on common ground, but each has irreducible aspects. Nevertheless what at last persuaded him was the fact, as it seemed to him, that psychology and physics had come very close: the new physics both of the atom and of relativistic space-time had effectively dematerialised matter, and psychology, especially in the form of behaviourism, had effectively materialised mind. From the internal viewpoint of introspection, mental reality is composed of sensations and images. From the external viewpoint of observation, material things are composed of sensations and sensibilia. A more or less unified theory therefore seems possible by treating the fundamental difference as one of arrangement: a mind is a construction of materials organised in one way, a brain more or less the same materials organised in another.

A striking feature of this view is, surprisingly, how idealist it is. Russell had, as noted, charged James with residual idealism. But here he is arguing something hardly distinguishable: that minds are composed of sensed percepts–viz. sensations and images–and matter is a logical fiction constructed of unsensed percepts. Now Russell had often insisted (using his earlier terminology) that sensibilia are 'physical' entities, in somewhat the sense in which, if one were talking about an item of sensory information in a nervous system, that datum would be present as impulses in a nerve or activity in a brain. But then nerves and brains, as objects of physical theory, are themselves to be understood as a constructions from sensibilia, not as traditionally-understood 'material substance', the concept of which physics has shown to be untenable. At the end of AMi (pp 305, 308) Russell accordingly says that 'an ultimate scientific account of what goes on in the world, if it were ascertainable, would resemble psychology rather than physics ... [because] psychology is nearer to what exists'. This explains Russell's notorious claim that 'brains consist of thoughts' and that when a physiologist looks at another person's brain, what he 'sees' is a portion of his own brain (Schilpp p 705).

For robuster versions of materialism this aspect of Russell's view is hard to accept. But it is not the only difficulty with his version of neutral monism. Not least among others is the fact that he failed in his main aim, which was to refute the view that consciousness is essential to the distinction between mental and physical phenomena. He had not of course attempted to analyse consciousness quite away; his aim was rather to reduce its importance to the mind-matter question. But images, feelings and sensations, which play so central a role in his theory, stubbornly remain conscious phenomena, whereas the sensibilia (by definition including unsensed sensa) which constitute the greater part of matter are not. Russell accepted this, but tried to specify a criterion of difference which did not trade on these facts, namely, the criterion of membership of different causal realms. But whereas that difference is open to question – and even if it exists might be too often hard to see – the consciousness difference is clear-cut. Relatedly, the intentionality which characterises consciousness cannot be left out of accounts of knowledge; memory and perception are inexplicable without it. Russell later acknowledged this point, and gave it as a reason in MPD for having to return to the question of perception and knowledge in later writings.

He also later came to abandon the idea – anyway deeply unsatisfactory from the point of view of a theory supposed to be both neutral and monist – that images and feelings are essentially mental, that is, not wholly reducible to neutral-stuff; for in a very late essay he says, 'An event is not rendered either mental or material by any intrinsic quality, but only by its causal relations. It is perfectly possible for an event to have both the causal relations characteristic of physics and those characteristic of psychology. In that case, the event is both mental and material at once'. This, for consistency, is what he should have argued in AMi 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 AMi, 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 AMi. 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 AMi which remain in his later thinking raise considerable difficulties for his views there about matter.

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