Psychologism is a generic type of position in philosophy according to which psychology plays a central role in grounding or explaining some other, non-psychological type of fact or law. The most common types 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 or explained 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).
John Stuart Mill seems to have been an advocate of a type of logical psychologism, as were many Nineteenth-Century German logicians such as Sigwart and Erdmann. Psychologism was famously criticized by Frege in his The Foundations of Arithmetic, and many of his works and essays, including his review of Husserl's Philosophy of Arithmetic. Edmund 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. The "Prolegomena" is considered a more concise, fair, and thorough refutation of psychologism than the criticisms made by Frege, and also it is considered today by many as being a memorable refutation for its decisive blow to psychologism.
There are other kinds of psychologism. For example, there is a psychologism pertaining to epistemology which mixes up problems of epistemology (context of justification) with problems of psychology (context of discovery). The criticism of this kind of psychologism can be traced back to Kant. Karl Popper criticized this kind of psychologism in his The Logic of Scientific Discovery, and some other works. According to Popper, one shouldn't confuse such questions as "How can a theory be tested?" (context of justification) with "How do we come up with a new theory?" (context of discovery). Psychologists, such as Locke or Hume, do not distinguish between these two questions.
psychologism in Czech: Psychologismus
psychologism in German: Psychologismus
psychologism in French: Psychologisme
psychologism in Hungarian: Pszichologizmus
psychologism in Russian: Психологизм
psychologism in Finnish: Psykologismi
psychologism in Swedish: Psykologism