What distinguishes neutral monism from its monistic rivals is the claim that the intrinsic nature of ultimate reality is neither mental nor physical. [...] Following a series of critical engagements with neutral monism (see especially Russell 1914a,b), Russell adopted it in Russell 1919 and remained a neutral monist for the rest of his long career: “I am conscious of no major change in my opinions since the adoption of neutral monism” is what he says in an interview from 1964 (Eames 1969: 108). [...]
For an entity to be neutral is to “have neither the hardness and indestructibility of matter, nor the reference to objects which is supposed to characterize the mind” (Russell 1921: 36; cf. 124). Russell never suspected sensations of being material (in this sense). That sensations are mental (in this sense)—that they consist of a mental act of sensing directed at a non-mental object—was, however, a pivotal part of his earlier view. But then his views changed:
“I formerly believed that my own inspection showed me the distinction between a noise [the object] and my hearing of a noise [the act of sensing], and I am now convinced that it shows me no such thing, and never did.” (Russell 1918b: 255)
Also, insofar as Moore and Russell are gesturing at similar issues, Moore’s paper provides some support for the claim that master-argument-ish reasoning was central to the idealism and rested on a simple error that was concealed by motivated reasoning and obfuscatory language, and that no one noticed (or successfully popularized) the error prior to Moore/Russell:
I am suggesting that the Idealist maintains that object and subject are necessarily connected, mainly because he fails to see that they are distinct, that they are two, at all. When he thinks of ‘yellow’ and when he thinks of the ‘sensation of yellow’, he fails to see that there is anything whatever in the latter which is not in the former. [...]
But I am well aware that there are many Idealists who would repel it as an utterly unfounded charge that they fail to distinguish between a sensation or idea and what I will call its object. And there are, I admit, many who not only imply, as we all do, that green is distinct from the sensation of green, but expressly insist upon the distinction as an important part of their system. They would perhaps only assert that the two form an inseparable unity.
But I wish to point out that many, who use this phrase, and who do admit the distinction, are not thereby absolved from the charge that they deny it. For there is a certain doctrine, very prevalent among philosophers nowadays, which by a very simple reduction may be seen to assert that two distinct things both are and are not distinct. A distinction is asserted; but it is also asserted that the things distinguished form an ‘organic unity’. But, forming such a unity, it is held, each would not be what it is apart from its relation to the other. Hence to consider either by itself is to make an illegitimate abstraction.
The recognition that there are ‘organic unities’ and ‘illegitimate abstractions’ in this sense is regarded as one of the chief conquests of modern philosophy. But what is the sense attached to these terms? An abstraction is illegitimate, when and only when we attempt to assert of a part—of something abstracted—that which is true only of the whole to which it belongs: and it may perhaps be useful to point out that this should not be done. But the application actually made of this principle, and what perhaps would be expressly acknowledged as its meaning, is something much the reverse of useful. The principle is used to assert that certain abstractions are in all cases illegitimate; that whenever you try to assert anything whatever of that which is part of an organic whole, what you assert can only be true of the whole. And this principle, so far from being a useful truth, is necessarily false. For if the whole can, nay must, be substituted for the part in all propositions and for all purposes, this can only be because the whole is absolutely identical with the part.
When, therefore, we are told that green and the sensation of green are certainly distinct but yet are not separable, or that it is an illegitimate abstraction to consider the one apart from the other, what these provisos are used to assert is, that though the two things are distinct yet you not only can but must treat them as if they were not. Many philosophers, therefore, when they admit a distinction, yet (following the lead of Hegel) boldly assert their right, in a slightly more obscure form of words, also to deny it. The principle of organic unities, like that of combined analysis and synthesis, is mainly used to defend the practice of holding both of two contradictory propositions, wherever this may seem convenient.
In this, as in other matters, Hegel’s main service to philosophy has consisted in giving a name to and erecting into a principle, a type of fallacy to which experience had shown philosophers, along with the rest of mankind, to be addicted. No wonder that he has followers and admirers. [...]
And at this point I need not conceal my opinion that no philosopher has ever yet succeeded in avoiding this self-contradictory error: that the most striking results both of Idealism and of Agnosticism are only obtained by identifying blue with the sensation of blue: that esse [“existing”] is held to be percipi [“being perceived”], solely because what is experienced is held to be identical with the experience of it. That Berkeley and Mill committed this error will, perhaps, be granted: that modern Idealists make it will, I hope, appear more probable later.
This updates me partway back toward the original claim I made (that Berkeley’s master argument was causally important for the rise of idealism and its 20th-century successors).
I should note that Russell was also led in some pretty weird directions by the desire to resist skeptical arguments:
Also, insofar as Moore and Russell are gesturing at similar issues, Moore’s paper provides some support for the claim that master-argument-ish reasoning was central to the idealism and rested on a simple error that was concealed by motivated reasoning and obfuscatory language, and that no one noticed (or successfully popularized) the error prior to Moore/Russell:
This updates me partway back toward the original claim I made (that Berkeley’s master argument was causally important for the rise of idealism and its 20th-century successors).