It is morally impossible, Locke argued, for individuals to consensually establish absolute rule over themselves. That would be to transfer to rulers a power that is not ours, but God’s alone: ownership of our lives. This article analyses the conceptual presuppositions of Locke’s argument for the moral impossibility of self-enslavement through a comparison with other classical social contract theorists, including Grotius, Hobbes and Pufendorf. Despite notoriously defending the permissibility of voluntary enslavement of individuals and even entire peoples, Grotius similarly endorsed divine ownership of human life. He could do so coherently, we show, because he denied that despotic power gives rulers rights in the lives of their subjects. Masters do not own slaves in the way we own material things (which we may destroy at will). Reworking received Roman law categories, Grotius maintained that ‘perfect slavery’ consists in masters having a personal right to the slave’s perpetual service; a condition equivalent to what Locke called ‘drudgery’ and deemed permissible. Our analysis of this unpalatable set of ideas reveals that Locke’s argument is premised upon idiosyncratic conceptions of slavery and absolutism, disavowed by prominent defenders of absolutism in the classical social contract tradition. It is hence less powerful than commonly believed.