 Consider first Frege’s Grundlagen der Arithmetik (1884). One of Frege’s main thesis is that mathematics and logic are not part of psychology, and that the objects and laws of mathematics and psychology are not defined, illuminated, proven true, or explained by psychological observations and results. One of Frege’s central arguments for this thesis is the consideration that whereas mathematics is the most exact of all sciences, psychology is imprecise and vague (1884, 38). Thus it is implausible to assume that mathematics could possibly be based upon, or be a part of, psychology. A closely related further point is that we need to distinguish between psychological ‘ideas’ (Vorstellungen) and their objects. This distinction is especially important when the latter are objective or ideal. Numbers, for example, are objective and ideal entities, and thus they differ fundamentally from ideas. Ideas are always subjective and idiosyncratic. In this context Frege laments the fact that the term ‘idea’ has also been used for objective, essentially non-sensual, abstract and objective entities (1884, 37). Frege rejects psychological or physiological interpretations of the Kantian distinctions between the a priori and the a posteriori, and the analytic and the synthetic; as Frege has, it these distinctions concern different ways in which judgments are justified or proven true, not different operations of the human mind (1884, 3). 