Economists generally argue that it is irrational to take more than cost-justified precaution against risks of accidental physical injury. Cost-justified precaution minimizes the dollars spent preventing and paying for accidents, thereby maximizing the wealth at society's disposal. When we take more than cost-justified precaution we ourselves worse off by making ourselves poorer. It costs more to prevent cost-justified accidents than it does to let those accidents happen and pay for the damage they do. Yet both common law and statutory risk regulation sometimes prescribe more than cost-justified precaution. Are these prescriptions irrational? This Article argues that they are not. When risks of devastating injury are imposed-when we risk premature death, or severe injury whose debilitating effects can never be fully undone-fairness generally requires more than cost-justified precaution. It is unfair to treat devastating injury as commensurable, at some ratio of exchange, with just any benefit which might be gained by risking such injury. Sacrificing an interest as urgent as the interest in avoiding premature death or devastating injury can only be justified if the burden of eliminating that risk is comparable to the burden of bearing it. This requirement of comparability means that we must usually take more than cost-justified precaution against risks of devastating injury. In this context, the Article examines two statutory norms which require more than cost-justified precaution-the feasibility and safety norms found in federal risk regulation. The feasibility norm calls for the elimination of significant risks of devastating injury, unless the elimination of those risks would prevent the activity which generates the risks involved from flourishing over the long run. The safety norm requires the elimination of all significant risks of devastating injury. The paper argues that feasible precaution is appropriate when an activity cannot flourish without imposing a significant risk of devastating injury, and the loss of the activity in question would work a harm comparable to and greater than the significant risk of devastating injury that is the price of the activity's flourishing. Safe precaution-the elimination of all significant risk of devastating injury-is appropriate when the benefits in question are not valuable enough for its elimination to count as a harm comparable to and greater than significant risk of devastating injury. I. THE PUZZLE: WHY PRESS PRECAUTION BEYOND THE POINT OF COST-JUSTIFICATION? A. The Clash Between Economic Science and Ordinary Moral Sensibility Years ago, Bruce Ackerman contrasted two competing perspectives on law, that of the ordinary observer and that of the scientific policymaker.1 The perceptions and discourse of the ordinary observer, Ackerman explained, start from the common practices and language of laymen.2 The scientific policymaker takes the realization of particular objectives-efficient precaution against risks of accidental injury and death, for example-as her end and uses the law as an instrument toward that end.3 Clashes between these two perspectives are endemic to our legal culture.4 Nowhere in the law of accidents is that conflict sharper than in cases where the risks imposed threaten severe and irreparable injury. A powerful and influential tradition of thought asserts that reasonable care in the law of negligence is, and ought to be, economically efficient care. When Learned Hand devised his famous formula for determining the amount of care due, Richard Posner argues, he was both adumbrating, perhaps unwittingly, an economic meaning of negligence, and attempting nothing more novel than to make explicit the standard that the courts had long applied.5 Judge Hand, as Robert Cooter and Thomas Ulen explain, set the legal standard of care by explicitly balancing the benefits and costs of precaution, just as an economist would have done . …