Neutral monism is the philosophical view that mental events and physical events are both to be reduced to aspects of some neutral stuff, which stuff considered by itself is neither physical nor mental.
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-physical-law-governed" rather than "strange.")
See also: Philosophy of mind, Double aspect theory , Dialectical monism