John Duns Scotus 1266-1308
John Duns Scotus was the founder of a Scholastic branch called Scotism. His philosophy, although Aristotelian in origin, had certain basic differences with that of Thomas Aquinas -- founder of the more popular Scholastic school Thomism.
Duns Scotus believed that God does not command an action because He thinks it is good, rather, an action is good because that action is performed by God. Scotus was one of the most profound and subtle of the medieval theologians and was given the title "the Subtle Doctor" (Doctor Subtilis).
Duns Scotus was a staunch defender of the concept of the Immaculate Conception, which was defined as dogma by Pope Pius IX in 1854.
His two best known works are On the First Principle and two commentaries
on the Sentences of the Italian theologian Peter Lombard.