 Psychologism is the view that social facts are composed exclusively out of the psychological states of individual people. This is the view advanced by Mill 1843 (see section  1.2  and also section   A.3.1 of the supplement    on the history of social ontology)  and endorsed by subsequent theorists of the “social mind” such as Tarde 1898, 1901. Economists in the late nineteenth and early twentieth centuries also advocated psychologism (Jevons 1871, Wicksteed 1910, Pareto 1916), as did mid-century social theorists (Popper 1945, Watkins 1952). The term ‘psychologism’ is a little confusing. Karl Popper, for instance, uses this word as a pejorative regarding a particular kind of methodology in social science. But when it comes to ontology, he endorses Mill’s view (Popper 1945).