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SCIENCE AND ITS LIMITS: THE REGULATOR'S DILEMMA 10 original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. Though this essay is subtitled "The Regulator's Dilemma," many of the same issues arise in the adjudication of disputes over who is to blame, and who is to be compensated, for damages allegedly caused by rare events. The regulator's dilemma is faced also by the toxic tort judgeâindeed the regulator's dilemma could equally be called the "toxic tort dilemma." If my car injures a pedestrian, I am liable to be suedâbut at issue is not whether I have injured the pedestrian; instead, the question is whether I am at fault for running into him. If the lead from my car's exhaust is alleged to cause bodily harm, the issue is not whether my car emitted lead but whether the lead actually caused the alleged harm. The two situations are quite different: in the first, the relation between cause and injury is not at issue; in the second, it is the issue. This paper, therefore, is an attempt to delineate more precisely those limits to science that give rise to the regulator's dilemma; I shall speculate on how these intrinsic limits to science seem to have catalyzed a profound attack on science by some sociologists and public interest activists; and I shall offer a few ideas that might help harried regulators finesse these trans-scientific limits of science. SCIENCE AND RARE EVENTS Science deals with regularities in our experience; art deals with singularities. It is no wonder that science tends to lose its predictive or even explanatory power when the phenomena it deals with are singular, unreproducible, and one of a kindâthat is, rareârather than regular, reproducible, and recurring. Though science can often analyze a rare event after the fact (say, the Cretaceous-Tertiary extinction), it has great difficulty predicting when such an uncommon event will occur. Let us distinguish between two sorts of rare eventsâ"accidents" and "low- level physical insults." Accidents are large-scale malfunctions whose etiology is not in doubt but whose a priori likelihood is very small. The occurrences at Three Mile Island in 1979 and at Bhopal, India, in 1984 are examples of accidents. The precursors to these events and the way in which the accidents unfolded are well understood. Estimates of the likelihood of the particular sequence of malfunctions are on less solid ground. As the number of individual accidents increases, prediction of their probability becomes more and more reliable. We can predict very well how many automobile fatalities will occur in 1986; we can hardly claim the same degree of reliability in predicting the number of serious reactor accidents in 1986. Low-level insults are rare in a sense different from "rare" as applied to accidents. We know that about 100 rads of radiation will double the mutation