Through the majority of the Middle Ages theologians and other intellectuals generally took for granted the idea that theology and philosophy were inherently compatible pursuits. Not until William of Ockham was that assumption to be seriously challenged. Though a deeply convicted theist, and an acute philosopher, Ockham held reservations on the capacity of philosophy to address the needs of theology.
William of Ockham was a student of Duns Scotus's philosophy. However, Ockham was skeptical about the efficacy of natural theology, and came to the conclusion that its arguments failed to prove what they claimed (Williams, 2007). He dismissed assumptions made by natural theologians such as the idea of an unmoved mover. Ockham challenged the idea that theology could be a science, since its first principles were not self evident. Consequently, theology and philosophy were not even the same kind of knowledge. Ockham believed that reason had limits; a person could reason from the best possible information available and still come to the wrong conclusions. Because reason and theology functioned differently, it was not necessary to try to reconcile them (Rubenstein, 2003). This position reiterates Dun Scotus's view that our understanding of the physical world is only probable, not certain.
Like Scotus, Ockham also advocated ontological parsimony. The way that Ockham and Scotus used parsimony was more semantic and metaphysical than the way modern scientists use it. Ockham set out to reduce metaphysics to the least number of entities possible. Where Aristotle saw ten categories of being, Ockham identified only two, substance and quality. According to Ockham's philosophy, it was possible that there were more entities than were logically necessary, but human reason could only defend the necessary, not the possible. While most medieval philosophers believed in universals, Ockham held a nominalistic position. He argued that reality consisted of particular experience, and from particular experience we can make generalizations. Opponents of Ockham's nominalism objected on the grounds that without universals, there was no objective reality, and the world is potentially unintelligible (Williams, 2007). Though Ockham was less confident in the ability of reason to lead to knowledge of God, he believed that there was a place for both faith and reason within the human experience.
By separating theology and philosophy, Ockham freed up natural philosophy from the need to conform to accepted religious beliefs. The parsimony espoused by Ockham and other medieval thinkers lead to an important methodological approach currently used in many scientific disciplines. The work of the scholastic philosophers helped integrate Aristotelian thinking into western thought. It is through the mediation of medieval philosophy and interpretation that modern thinkers view Aristotelian ideas. Though many aspects of medieval and Aristotelian thinking were eventually rejected by later thinkers, the legacy of the Middle Ages offered many useful tools for the engagement of modern science.
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