Marshall's definition of economics as “a study of mankind in the ordinary business of life” is hopelessly ambiguous; but it effectively highlights one central difference between economics and physical sciences: The subject matter of economics is the stuff of everyday experience. Unfortunately, familiarity sometimes encourages us to treat familiar facts as self-evident truths rather than habits of mind and so unconsciously to inject these “truths” into formal theoretical arguments where they do not belong. Outwardly valid theoretical findings that emulate superficial technical aspects of exact sciences then become problematic because they derive not from explicit assumptions but rather from a confusion of habits of thought with rational analysis (cf. Born, 1962: 226). Because the confusion thus introduced is a matter of conceptual dissonance rather than logical inconsistency, the vacuousness of much supposedly “settled” theory is thus masked.