A comparison of two U.S. Supreme Court cases about fundamental rights, one on slavery, the other on abortion, sheds light on constitutional law and the principles undergirding liberal constitutional democracy. The Dred Scott case in 1857 denied constitutional rights to enslaved Africans and their descendants living in the United States. The Roe v. Wade decision in 1973 created a constitutional right to abortion that denied constitutional personhood to human beings prior to birth. Both cases involved applications of what legal scholars call “substantive due process”—that is, a substantive interpretation of the constitutional requirement that governments provide persons with “due process of law” that moves beyond procedural formalism. Although many constitutional scholars deny the legitimacy of substantive due process as a legal doctrine, this article proposes that the judicial system cannot ultimately avoid substantive moral questions in constitutional interpretation. In both cases examined here, the crucial question was about who counts as part of the people whom the Constitution protects, and that question could not be answered in purely formal terms. Both Dred Scott and Roe v. Wade erred not by engaging substantive moral questions but rather by denying, in different ways, the natural rights of human persons.