 Russell appears to have developed this theory around 1913, while working on his Theory of Knowledge manuscript and on his 1914 Monist article, “On the Nature of Acquaintance.” Decades later, in 1964, he remarked that “I am not conscious of any serious change in my philosophy since I adopted neutral monism” (Eames 1967, 511). Even so, over the next several decades Russell continued to do a large amount of original work, authoring such important books as The Analysis of Mind (1921), The Analysis of Matter (1927a), An Inquiry into Meaning and Truth (1940) and Human Knowledge: Its Scope and Limits (1948).