In The Possibility Condition, Annabel Brett offers a new analysis of early modern rights and resistance by reading the relationship between rulers and subjects in terms of the 'possibility condition', or the idea that law must be possible for people to obey. Beginning in classical antiquity, the book traces the possibility condition through its medieval and renaissance reception to the Reformation, when the thesis that God's law is impossible for human beings to obey proved theological dynamite. Theologians of the Counter-Reformation responded with the concept of moral impossibility and its converse, moral possibility. From here Brett shows how widespread was the idea of law's possibility in early modern political thought, and argues that the idea of moral possibility as a condition of law fundamentally shapes the relationship between liberty and authority in a series of key political theorists including Francisco Suárez, Hugo Grotius, and Thomas Hobbes. Two chapters further explore how it was deployed in the practical contexts of plague time in early modern Europe and colonial government in Mexico and Peru. Overall, Brett suggests that putting the possibility condition back in makes better sense not only of some of the most influential political theories but of the broader political, legal, and religious culture of the time.