The Article sets forth a conclusive answer to one of the most fundamental questions in tort law, which has bedeviled and divided courts and scholars for centuries: Should reasonableness be a normative or a positive notion? Put differently, should the reasonable person be defined in accordance with a particular normative ethical commitment, be it welfare maximization, equal freedom, ethic of care, and so forth, or in accordance with an empirically observed practice or perception? Only after answering this question can one move on to selecting a concrete definition of reasonableness. Our own answer is radical but inescapable: Only normative definitions are logically acceptable. The Article does not endorse a particular definition of reasonableness. Instead, it focuses on the fundamental choice between the two conflicting paradigms. We put forward and defend the thesis that normative definitions are categorically preferable to positive definitions, because the latter are logically unacceptable, whereas the former merely raise partially surmountable practical problems. Although the Article focuses on the reasonable person in torts, the implications of our analysis are far-reaching, because the concept of reasonableness prevails in most areas of American law.