The predominant school of thought in the 13th century was the Thomistic reconciliation of the teachings of Aristotle with Christian theology. The Condemnation of 1277, enacted at the University of Paris, placed restrictions on ideas that could be interpreted as heretical; restrictions that had implication for Aristotelian thought. An alternative was presented by William of Ockham, who insisted that the world of reason and the world of faith had to be kept apart. Ockham introduced the principle of parsimony – or Occam's razor – whereby a simple theory is preferred to a more complex one, and speculation on unobservable phenomena is avoided.
Is there an answer to this question (If it cannot be answered, say "unanswerable"): Which philosopher believed that reason and faith were to be kept separate?
William of Ockham