John Duns Scotus was born c.1266 in the small town of Duns, just north of the border between England and Scotland, and some time early in his life became a Franciscan friar. By inference from the place of his ordination in 1291 (Northampton), we learn that he was studying at Oxford by that date. Scotus remained in Oxford until at least 1301, and in the last couple of years of the thirteenth century started lecturing on Peter Lombard's Sentences, a necessary step for a Bachelor of Theology on the way to becoming a Master of Theology. We know that Scotus was in Paris, lecturing on the Sentences, during the academic year 1302–3, in order to qualify as Master of Theology in the pre-eminent of the two great medieval theological centres. He became Master of Theology at Paris in 1305, and was moved to Cologne in 1307. Known from very soon after his death in 1308 as the ‘subtle doctor’, Scotus wrote the first systematic treatise dedicated to a proof for God's existence, the De primo principio or On the First Principle. As his nickname might suggest, the treatise – as with most of Scotus' works – is not an easy read. In what follows, I shall try to summarize some of the moves that Scotus makes in this treatise, and add some further relevant material from other works of Scotus'.