Psychologism is the view that mathematical sentences and theories are about mental objects. Probably the most common version of this view holds that numbers are something like ideas in our heads, and ordinary mathematical sentences like ‘3 is prime’ provide descriptions of these ideas. This view was popular in the late 19th Century; it was endorsed by, e.g., the early Husserl (1891), as well as the intuitionists, Brouwer (1912, 1948) and Heyting (1956). But Frege (1884, 1893–1903) provided a hos