This book is a lively and highly accessible introduction to the thought of Thomas Aquinas. While primarily a theologian, Aquinas' conception of theology presupposed an autonomous philosophy. This book concentrates on his philosophy while making clear its openness to theology as reflection on Revelation. As a philosopher, Aquinas is fundamentally Aristotelian. Like Aristotle, he sees philosophy as emerging from the ordinary thinking of ordinary human beings (and philosophers when they are off duty). Philosophy does not initiate certain knowledge but prolongs it by perfecting the instrument of thinking and expanding its content. The quest for wisdom, like that for happiness, is an inescapable fact of human existence. This book uses key and crucial texts to describe the trajectory of Aquinas' philosophical thought from the analysis of changeable things through the reasoned awareness that to be and to be material are not identical to such knowledge as we can have of God. This brings Aquinas to the threshold of Christian faith.