Thomas Aquinas (1224–1274) redefined faith as a kind of knowledge, rather than as a specific feeling or attitude of mind. As such, he said that faith fell between opinion and scientific knowledge. Faith was greater than opinion because it involved strong agreement, as an act of will, and it was less than scientific knowledge because it lacked factual evidence that could compel agreement.

Aquinas thought that philosophy was reasoning based on existing knowledge or experience, leading to new knowledge, which he called “the way of discovery.” He held that philosophy was also the use of reason to confirm beliefs by tracing them back to basic principles, which he called “the way of reduction.” Philosophy becomes theology if the beliefs one begins with are based on faith. There are, in turn, two kinds of theology: truths in Scripture that are learned for their own sake, and metaphysics or explanations based on religious principles.

Despite his theological idea of metaphysics, Aquinas did distinguish between philosophy and theology. For instance, in De Aeternitate Mundi, although he held the religious belief that the universe was not eternal, he said that it might be eternal based on philosophical reasoning. In general, apart from religious revelation, Aquinas believed that we get our knowledge from sense experience and our intellectual understanding of our sense experience.