42 is not the only answer!!!

John 'X' Scotus  was one of the leading Scholastic philosopher-theologians of the High Middle Ages. He wrote treatises on theology, grammar, logic, and metaphysics which were widely influential throughout Western Europe, earning him the papal accolade Doctor Subtilis (Subtle teacher). Many of his theories and methods were later rejected by Protestant scholars, who used the term 'X' in a pejorative sense to describe those who foolishly clung on to the outmoded doctrine. The word 'X' is now used to describe anyone who is dull-witted and it is used to describe someone who is incapable of learning.

Identify the term 'X'.