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56 International Security, Vol. 24, No. 2 (Fall 1999), pp. 56–73© 1999 by the President and Fellows of Harvard College and the Massachusetts Institute of Technology. Sorting Through the Wealth of Notions Bruce Bueno de Mesquita and James D. Morrow The opportunity to better inform the readership of International Security on the important contributions of a rational choice perspective is most welcome. We and Stephen Walt agree on many issues. He says that “[social science] requires theories that are . . . logically consistent,”1 and that “formal techniques facilitate the construction of precise and deductively sound arguments” (p. 8). Walt asserts, correctly we believe, that “the formal language of mathematics can impart greater precision to an argument, and helps guard against inconsistencies arising either from a failure to spell out the causal logic in detail or from ambiguities of normal language” (p. 14) and that the “virtues [of formal theory] should not be dismissed lightly” (p. 15). We agree completely with these views. Walt raises issues worthy of fuller discussion. He contends, and we agree, that “the central aim of social science is to develop knowledge that is relevant to understanding important social problems. Among other things, this task requires theories that are precise, logically consistent, original, and empirically valid” (p. 8). We discuss how the rational choice approach to security studies contributes signiªcantly in these ways. Additionally, we address some misrepresentations in Walt’s article. The Centrality of Logical Consistency for Scientiªc Theories Walt gives three criteria for evaluating social science theories: logical consistency , degree of originality, and empirical validity. We believe that logical consistency takes precedence over the other two criteria; without logical consistency , neither the originality of a theory nor its empirical validity can be judged. Logical consistency is the ªrst test of a theory because consistency is necessary, though not sufªcient, for understanding how international politics works. Bruce Bueno de Mesquita is Senior Fellow at the Hoover Institution, Stanford University. He is ªnishing a textbook, Principles of International Politics, to be published by Congressional Quarterly Press later this year. James D. Morrow is Senior Research Fellow at the Hoover Institution, Stanford University. We thank Robert Powell and Frank Zagare for their comments on an earlier draft. 1. Stephen M. Walt, “Rigor or Rigor Mortis? Rational Choice and Security Studies,” International Security, Vol. 23, No. 4 (Spring 1999), pp. 5–48, at p. 8. Subsequent references to Walt’s article appear in the text. A basic point in logic drives our view. A theory, in terms of logic, consists of a system of assumptions and conclusions derived from those assumptions. A logical inconsistency exists when two mutually contradictory statements can be derived from the assumptions of a theory. When such a contradiction exists in a theory, then any statement follows logically from the theory. There is, then, no discipline for arguments in a logically inconsistent theory; those using the theory are free to draw any conclusion they wish from the premises of the theory. Logical inconsistencies deny the possibility of a theory having empirical content. Theories derive empirical content by producing falsiªable hypotheses, conclusions that could be contradicted by evidence. A theory gains credence as more of its falsiªable propositions are supported by evidence, although there are no hard and fast rules here. However, because any pattern of evidence can be matched with some conclusion of a logically inconsistent theory, such theories cannot be falsiªed and so cannot have empirical content. A theory is falsiªed when an alternative is shown to ªt the range of predictions better than the initial theory. Falsiªcation of a theory cannot happen if any evidence can be interpreted as an implication of the theory. Theories with logical inconsistencies can also appear highly original simply because there are no constraints on reaching conclusions. Such a theory appears to “explain” all previous results while also allowing its proposer to advance any claims that appear to reºect the historical record as she sees it. The originality of a logically inconsistent theory is dubious at best. Further, logically inconsistent theories present serious problems for policy prescriptions, a central goal of social science theory according to Walt. Again, any...


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