Blessed John Duns the Scot was a thirteenth century Franciscan who studied at Oxford and then at the University of Paris. He left without a Master's degree, but lectured in theology at Oxford and Cambridge anyway. [Since I don't have highly qualified teaching status for two of the three subjects I'm teaching this year, I'm liking this John fella already.]
Back in France, he ran afoul of the king, fled, and needed a letter of recommendation to get back. His former teacher, Gonsalvus Hispanus, wrote the letter, praising his "most subtle genius." This led to his moniker doctor subtilis, the Subtle Doctor. [Okay, so I'm thinking we parted ways with that paragraph. Actually, teaching high school American lit is a little less remarkable than teaching theology at the University of Paris, but I enjoyed the parallel, even if it was on a different plane.]
John was working on a universal understanding that would include all knowledge, even though conservative theologians had already been pounding Aquinas' work for over-reliance on Aristotle. [True? I'm taking Butler's word for it.] His universal theory included all the great Christian writers and even the (Church-friendly) writings of Avicenna, the great Muslim scholar. Oddly enough, that wasn't the controversial part of his work.
John Duns Scotus (according to Butler) was the author of the doctrine of the Immaculate Conception, the notion that Mary Theotokos was born without original sin. This special status of hers meant two things: first, she was uniquely qualified to be the mother of the savior, and second, she was pre-redeemed, i.e. saved without the sacrifice of the Crucifixion. It became such a widely accepted view that most folks don't even understand the term Immaculate Conception anymore, believing that it refers to the conception of Jesus without sexual intercourse. Of course that's a more remarkable miracle than a conception without original sin, but then again, he was called Doctor Subtle.