Should the historical origins of some principle or practice affect how we think about it today? Under one standard view, the answer is “no”; to think otherwise is to commit a fallacy – specifically, the “genetic” fallacy. But in legal argument, origins often seem to matter a great deal. This essay takes up the question of whether, or under what conditions, it is right for them to do so. It shows that under at least three models of legal reasoning — reasoning by authority, reasoning for the sake of integrity, and a third form of reasoning that I call ad hominem argumentation — historical explanations are properly relevant to the legal analysis. Legal theorists have yet to even recognize this third, ad hominem model of reasoning as a distinct form of legal argument. But they should. It not only best accords with one very traditional understanding of the common law; it also explains the logic and appeal of a “living constitution,” as the Supreme Court conveyed in one of its most controversial decisions of the last few decades: Planned Parenthood of Southeastern Pennsylvania v. Casey.