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philosophical (i.e., conventionalist) criteria but rather they, too, are empirically based, hence can be expressed in instrumentalist terms: Simpler means requires less empirical knowledge (the word initial refers here to the process of generating predictions with something like modus ponens). More fruitful means more applicable and more precise [4, p. 10]. The possibility of a tradeoff is discussed. Friedman explicitly rejects the necessity of requiring the of substantive hypotheses before they are used simply because it is possible. But here it should be noted that his rejection of is partly a consequence of his use of the word testing. Throughout his essay always means testing for (in some sense). It never means testing in order to reject as most of his critics seem to presume. That is, for Friedman a successful test is one which shows a statement (e.g., an assumption, hypothesis, or theory) to be true; and, of course, a minimum condition for a successful test is that the statement be inconsistent with empirical evidence (see [4, pp. 33-34]).14 Appreciating the success orientation of Friedman's view is essential to an understanding of his methodological judgements. For Friedman, an instrumentalist, hypotheses are chosen because they are successful in yielding true predictions. In other words, hypotheses and theories are viewed as instruments for successful predictions. It is his assumption that there has been a prior application of modus tollens (by evolution, see [4, p. 22]), which eliminates unsuccessful hypotheses (ones that yield false predictions), and which allows one to face only the problem of choosing between successful hypotheses. In this 13 Note here, although Friedman uses conventionalist criteria, it is for a different purpose. For a conventionalist the criteria are used as status substitutes; in conventionalism one finds that theories are either or worse. In this sense, Friedman can be seen to pose the problem of choosing among theories already classified as better in his sense (successful predictions). 14 I stress, this is the view Friedman uses in his essay. In recent correspondence Professor Friedman has indicated to me his more general views of in which success might be either a confirmation or a disconfirmation. But he still would question the meaningfulness of testing in order to reject. Although Friedman seldom uses the word truth, it should be noted that throughout he consistently uses the word (by which he always means at least not inconsistent with the available facts) in the same sense that truth plays in modus ponens seemingly while also recognizing that modus ponens is assured only when applied to truth in the absolute or universal sense (i.e., without exceptions). Technically speaking his use of the word may lead one to the incorrect identification of truth with validity. In this regard, applications of Friedman's methodology are often confused with orthodox conventionalism. This confusion can be avoided by remembering that is a (but sufficient) condition of empirical truth-hence, validity and are identical-and by recognizing that someone can believe his theory is true, even though he knows he cannot prove that it is true. This content downloaded from 40.77.167.14 on Tue, 20 Sep 2016 06:14:18 UTC All use subject to http://about.jstor.org/terms 512 Journal of Economic Literature, Vol. XVII (June 1979) sense, his concentrating on successful predictions precludes any further application of modus tollens. And similarly, any possible falsity of the is thereby considered irrelevant. Such a consideration is merely an appreciation of the logical limitations of what I called reverse modus tollens (above, Section 1.2). And since he has thus assumed that we are dealing exclusively with successful predictions (i.e., true conclusions), nothing would be gained by applying modus ponens either. This is a straightforward appreciation of the limitations of what I called reverse modus ponens. Knowing for sure that the hypotheses (or assumptions) are true is essential for a practical application of modus ponens, but such knowledge, he implies, is precluded by the absence of an inductive logic [4, pp. 12-14]. By focusing only on successful hypotheses, Friedman correctly reaches the conclusion that the application of the criterion of simplicity is relevant. He says there is virtue in a simple hypothesis if its application requires less empirical information. One reason a simple hypothesis can require less information, Friedman says, is that it is descriptively false [4, pp. 14-15]. (For example, a linear function requires fewer observations for a fit than does a quadratic function.) This raises the question of descriptions versus necessary abstractions. Friedman explicitly recognizes that some economists (presumably, followers of Lionel Robbins) hold a view contrary to his. For them the of a theory is considered to be a direct result of the descriptive realism of the assumptions. But Friedman claims the relation between the significance of a theory and the realism of its assumptions is almost the opposite. . . . Truly important and significant hypotheses will be found to have assumptions that are wildly inaccurate descriptive representations of reality, and, in general, the more significant the theory, the more unrealistic the (in this sense)....
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