What makes a law just—and what makes it possible? In a trio of thought-provoking lectures from the Tanner Lecture on Human Values series, Cambridge University political theorist Annabel Brett examines how the boundary between legal duty and human capacity has shaped political thought for centuries. Drawing on sources from classical antiquity to early modern Europe, Brett explores the idea of moral possibility: the principle that laws must demand only what people can realistically or ethically be expected to do. Through stories of tyranny and virtue, resistance and responsibility, she shows how this idea remains central to today’s debates on justice, democracy, and social obligation.

Subjects and Citizens: The Possibility Condition, Law and Democracy

In this lecture, Brett traces the deep roots of the idea that demanding the impossible is a form of tyranny. From ancient Greece to Renaissance Europe, legal and political thinkers argued that for laws to be just, they must be within people’s power to follow. A striking example comes from the Bible, where Pharaoh forces the Israelites to make bricks without straw—an archetype of unjust command. Brett explores how this “possibility condition” served as a foundation for later democratic thinking, influencing the development of fairer, more humane legal systems.

Times of Change: Possibility, Virtue and a Democratic Politics of Time

Here, Brett dives deeper into how what is considered “morally possible” changes with time, culture, and circumstance. She explains that while law can ask more of people during times of crisis—such as war or plague—those demands must still be grounded in real human capabilities. Brett critiques how colonial powers twisted this concept to justify oppressive practices, such as the Spanish use of forced labor in Peru. In contrast, she highlights more democratic models, such as Domingo de Soto’s recognition of the poor’s right to beg. The lecture offers a compelling case for time-sensitive, human-centered justice.

The Times of Possibility

Bringing the series full circle, this conversation brings together key insights from previous lectures to show how “moral possibility” operates across history and into the present. Drawing on thinkers like Aquinas, Suarez, and Hobbes, Brett considers how people navigate the space between what is legally required and what is humanly possible. With input from Melissa Lane and David Dyzenhaus, the discussion explores real-world implications—from the ethics of military service and civil disobedience to climate policy and political resistance. At its heart is a powerful question: How do time, culture, and institutions shape what we can—and should—be asked to do?

Together, these videos invite us to rethink what justice demands. They show that the fairness of a law isn’t just about its content, but about whether it honors the limits of human capacity. As laws evolve alongside new social, environmental, and political challenges, Brett’s work reminds us that legal systems must remain rooted in moral possibility—balancing aspiration with empathy, and obligation with understanding.

Explore these programs and more in the Tanner Lectures on Human Values series.