 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). But the question whether Russell’s neutral monism is best seen as a single theory or as a sequence of related but significantly different theories, as well as the question which, if any, of these different doctrines should count as versions of neutral monism, have been much debated (see Wishon 2015; Bostock 2012; Stace 1946). Russell’s 1919 and 1921 are generally considered to represent the early versions of his neutral monism. Russell’s 1927a and 1927b contain the mature doctrine. In his later works—Russell 1948 and 1956a—he no longer used the term “neutral monism”. But the doctrine seems largely unchanged.