 Aristotle and many of his followers believed they could prove that the world had always existed. Conversely, many of Aquinas’s contemporaries thought they could prove the opposite, that the world had to have a beginning in time, and the debate between these rival philosophical accounts was one of the most acrimonious issues in the early medieval university (Dales 1990). Aquinas took up the question in detail in his treatise On the Eternity of the World, concluding that no demonstrative argument is possible either way. For all we can tell, God could have created the world in such a way that it had always existed. That we think otherwise is another deliverance of the faith, not susceptible to philosophical proof. This measured conclusion is characteristic of his thought in many domains. On the one hand, he has complete confidence that philosophical reasoning, properly pursued, will not yield results that are a threat to Christianity. On the other hand, he thinks it discredits the faith to defend it through dubious attempts at demonstration. The key is to see where philosophy can be of service, and where it must give way to revealed doctrine (Wippel 1995c; Kretzmann 1999 ch. 5).