The main philosophers of the Middle Ages were religious thinkers; St 奥古斯丁 (354- 430 CE) and 托马斯·阿奎那 (1225-1274 CE) for Christianity and Averroes ( 1126-1198 CE) for Islam.
奥古斯丁’s main preoccupation was reconciling the existence of evil with the notion of a 神 that was all powerful and all merciful. He claimed to have resolved this apparent contradiction through his idea that 神 had given human beings 自由意志 so that they could choose between good and evil. 这样，人类就给自己带来了邪恶，因为我们积极地选择了物质世界中易受腐蚀的元素，而不是永恒的，完美的形式，它们是精神的。
Despite his views on 自由意志, 奥古斯丁 believed that 神 had already pre-determined (before people are born) those who will be saved and go to heaven. He also believed that due to Adam and Eve’s disobedience of 神 in the Garden of Eden, all human beings are born as wrong-doers or sinners. This is the notion of “original sin”.
因此，奥古斯丁的观点似乎缺乏连贯性。很难理解“free will” if all human beings are “born bad” and if 神 has already determined who will be saved before a person even has a chance to exercise 自由意志 通过 choosing between good and evil. Perhaps Augustine’s 哲学 has no real point apart from the attempt to reconcile the notion of a 神 who is supposed to be all powerful and all merciful with the obvious existence of evil.
奥古斯丁’s other important view, in a historical sense, was that the relationship between human beings and 神 must be managed through the Catholic Church. This view dominated 宗教, and indeed the whole structure of society, throughout the Middle Ages. It lasted until Luther and the advent of Protestantism which taught that individuals could form their own direct relationships with 神.
阿维罗埃支持亚里士多德反对新柏拉图主义的经验主义，并辩称亚里士多德的哲学与宗教之间没有不相容的地方。无论《古兰经》在哪里与理性方法不相容，阿维罗伊斯都认为应该以非文字，隐喻的方式来理解它。但他还相信，只有受过良好教育的学者才能理解诗歌的隐喻，因此，普通百姓应该对古兰经的字面解释保持满意，以免他们感到困惑。具有讽刺意味的是，这反映了柏拉图的观点，即普通百姓就像一个山洞里的囚徒，无法理解“true” nature of reality.
From Plato he derived the notion of the “essence of things”. Thus we can conceive of the essence of a dragon but still deny its existence. He argued that as 神 designed everything the essence of a thing must precede its existence. But he also took from Aristotle the idea that we acquire knowledge through the senses.
阿奎那发展了他的五个“proofs” for the existence of 神 from Aristotle’s notion of causes. He argued, for example that the universe has a plan (Aristotle’s formal cause) must have a maker or first cause, being 神 (Aristotle’s efficient cause) and that everything in the universe works to a purpose, namely 神 (Aristotle’s final cause).
阿奎那（Aquinas）也将亚里士多德（Aristotle）解释为支持存在“natural law”. Aquinas conceived of natural law existing in a separate realm to human law. Natural law was seen as reflecting the purposes that 神 must have built into things.
因此，阿奎那认为“Whatever is a means of preserving human life and of warding off its obstacles belongs to 自然法则;自然规律”.
More certainly it is the notion of 自然法则;自然规律 that lies behind Catholic doctrinal teaching against homosexual acts, masturbation and oral sex. All of these are seen as use of genitalia contrary to the “natural purpose” for which 神 created them, namely procreation.