This article explores silence and the ethics of conscience through a study of the life and late letters of Sir Thomas More. More's silence is a paradigm case for the contest of conscience under conditions of tyranny, and it is one of the most influential in the formulation of principles of conscience and human rights in modern constitutional democracies. The article focuses on More's prosecution by King Henry VIII, and offers close analysis of the concept of conscience discussed in his last letters. The approach is historical and philosophical, and puts More's case in dialogue with other classic and modern texts on the ethics of conscience. Emphasis is placed on More's principle of self-determination as essential to conscience.