In his admirable book on The Scope and Method of Political Economy John Neville Keynes distinguishes among “a positive science … [,] a body of systematized knowledge concerning what is; a normative or regulative science … [,] a body of systematized knowledge discussing criteria of what ought to be …; an art … [,] a system of rules for the attainment of a given end”; comments that “confusion between them is common and has been the source of many mischievous errors”; and urges the importance of “recognizing a distinct positive science of political economy.”

This [essay] is concerned primarily with certain methodological problems that arise in constructing the “distinct positive science” Keynes called for – in particular, the problem how to decide whether a suggested hypothesis or theory should be tentatively accepted as part of the “body of systematized knowledge concerning what is.”