Neutral monism was introduced by the famous 17th century Dutch philosopher, Baruch Spinoza, and a version of it was propounded at various times by Bertrand Russell.
The American philosopher Donald Davidson advanced a position on mind-body identity he called "anomalous monism," which is related to but probably not exactly the same as these earlier theories. ("Anomalous" here meaning "not-law-governed" rather than "strange.")
See also: Philosophy of mind, Double aspect theory