This is an accessible introduction to the life and thought of John Duns Scotus (c. 1266--1308), the scholastic philosopher and theologian who came to be called the Subtle Doctor. A native of Scotland (as his name implies), Scotus became a Franciscan and taught in Oxford, Paris, and Cologne. In his writings he put Aristotelian thought to the service of Christian theology and was the founder of a school of scholasticism called Scotism, which was often opposed to the Thomism of the followers of Thomas Aquinas. In particular, Scotus is well known for his defense of contra-causal free...