For years, Thomists have debated Aquinas’s view on the status of human beings or persons between death and resurrection. Survivalists hold that, for Aquinas, the survival of the separated soul is sufficient for the continued existence of the human being; corruptionists deny this, insisting that the body is also necessary for a human being to exist, absolutely speaking. Most survivalists agree that matter is part of a human being’s nature, signified by its essential definition. So how can a human being survive the loss of its body at death? Many survivalists reply that a thing’s essence and definition only express what it is naturally, or normally, or typically, but not necessarily. In this paper, I argue that this view of essences and definitions is not Aquinas’s own. This comes out clearly in Aquinas’s treatments of God’s absolute power, which he thinks is limited only by logical contradiction. In such treatments, Aquinas consistently appeals to the natures of things to explain why not even God can make things to be other than they are by definition, on pain of logical contradiction. This shows that he thinks of a thing’s essence and definition as strictly necessary, not merely normative, for its existence.