Psychologism is a philosophical position, according to which psychology plays a central role in grounding or explaining some other, non-psychological type of fact or law. The Oxford English Dictionary defines psychologism as: "The view or doctrine that a theory of psychology or ideas forms the basis of an account of metaphysics, epistemology, or meaning; (sometimes) spec. the explanation or derivation of mathematical or logical laws in terms of psychological facts." Psychologism in epistemology, the idea that its problems "can be solved satisfactorily by the psychological study of the development of mental processes", was argued in John Locke's An Essay Concerning Human Understanding (1690).
Other forms of psychologism are logical psychologism and mathematical psychologism. Logical psychologism is a position in logic (or the philosophy of logic) according to which logical laws and mathematical laws are grounded in, derived from, explained or exhausted by psychological facts (or laws). Psychologism in the philosophy of mathematics is the position that mathematical concepts and/or truths are grounded in, derived from or explained by psychological facts (or laws).
The word was coined by Johann Eduard Erdmann as Psychologismus, being translated into English as psychologism.
John Stuart Mill was accused by Edmund Husserl of being an advocate of a type of logical psychologism, although this may not have been the case. So were many nineteenth-century German logicians such as Christoph von Sigwart and Erdmann himself, as well as a number of psychologists, past and present (e.g., Gustave Le Bon). Psychologism was criticized by Gottlob Frege in his The Foundations of Arithmetic, and many of his works and essays, including his review of Husserl's Philosophy of Arithmetic. Husserl, in the first volume of his Logical Investigations, called "The Prolegomena of Pure Logic", criticized psychologism thoroughly and sought to distance himself from it. Frege's arguments were largely ignored, while Husserl's were widely discussed.
In "Psychologism and Behaviorism", Ned Block takes psychologism as the position that "whether behavior is intelligent behavior depends on the character of the internal information processing that produces it." This is in contrast to a behavioral view which would state that intelligence can be ascribed to a being solely via observing its behavior. This type of behavioral view is strongly associated with the Turing test.