Some would say that psychology is concerned with mental phenomena, but they would be puzzled if they were required to state in what respect, if any, "mental" phenomena differ from those which provide the data of physics. Fundamental psychological questions quickly take us into regions of philosophical uncertainty, and it is more difficult than in other sciences to avoid fundamental questions, because of the paucity of exact experimental knowledge.
Source: Religion and Science, 1935, by Bertrand Russell
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