Where a medieval thinker thought an idea, whether coming from a pagan, Muslim, or Jew, had reason on his side, he would accept it and incorporate it into his own scheme of thought. Where he thought a non-Christian philosopher was wrong, especially where the thinker argued for something contrary to Christian doctrine or something which implied such, the Christian would argue against him. But as often as not the Christian would not refute the infidel using the Bible, the pope, or some other Christian authority, but using the principles of the infidel philosophers themselves! I know firsthand of many, many cases where scholastics argue that Aristotle or whoever was wrong about such-and-such given Aristotle’s own principles, and where he came to a conclusion incompatible with Christianity, this is not simply because he lacked the True Faith, but also and especially because he had failed as a philosopher to discover the best arguments available to reason on the subject.
To use an image they themselves loved to reproduce, the medievals saw themselves as the Jews during the Exodus, who as they were leaving Egypt for the promised land despoiled the Egyptians of the riches owed to them for their generations of servitude (i.e. they claimed reparations). The riches of Truth for them came from God, and properly belonged to those who were God’s friends and faithful servants. If the pagans and infidels had come into possession some truth on their own, it belonged with just as much right to Christianity as well, and so Christians would appropriate good reasons and good arguments wherever they found them.
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