Benjamin Zipursky’s Reasonableness In and Out of Negligence Law is a characteristically thoughtful, sensible, and learned discussion of the idea of reasonableness in and out of negligence law. As he rightly observes, his views are very similar to my own. Except in one respect. For Professor Zipursky, the Hand Formula is the Lord Voldemort of negligence law. It looms over the law of negligence, but it must not be named. Indeed, it must be killed, or at least banished. We part ways on this point and I confess to finding Professor Zipursky’s vendetta perplexing. Trying to write the Hand Formula out of negligence law at this late date is tantamount to repudiating one hundred years of tort law and theory. This revisionary theorizing is as unnecessary as it is quixotic. The Hand Formula is not only too deeply embedded in negligence law to uproot; it is also unobjectionable. Indeed, the Hand Formula is one of modern negligence law’s more important achievements. It frees the concept of due care from a disturbing identification with customary practice and ties due care more closely to the idea of justified conduct. By doing so, it gives negligence law an important critical dimension.
The Hand Formula draws its deep appeal from two sources. First, as Landes and Posner observe, it has the feel and smell of an exercise in making explicit what had long been implicit in negligence cases. Second, it strikes us as a cogent distillation of considerations that are plainly relevant to determining whether due care has been exercised. When we ask if a defendant should have acted as she did—or should instead have exercised more care—it is eminently sensible to inquire into the magnitude and probability of the harm that the defendant risked and the burden of avoiding that harm. Economic and philosophical theories of negligence are eager to claim the Hand Formula as their own because it is both a central construct of negligence law and independently attractive. If there were no Hand Formula, we would have to invent it. To be sure, the economic theory of the Hand Formula is a different matter. Professor Zipursky and I both think that it is deeply flawed. The economic interpretation, however, is not the Hand Formula itself. It goes well beyond the Hand Formula in important and questionable ways. The economic interpretation of the Hand Formula imposes a debatable theory of social choice on a conceptual tool. In so doing the economic interpretation makes claims about the commensurability of harms and the cost of their avoidance that Hand himself explicitly rejected.