Book excerpt: Thomas Aquinas is generally acknowledged to be the greatest theologian of the Middle Ages and his masterpiece, 'Summa Theologica', provides a complete and authoritative statement of medieval economic thought that has remained the official Catholic view right up to the present time. St Thomas had a decisive influence on economic thought in at least three broad areas: the theory of private property, the theory of the just price and the doctrine of usury. St Thomas's great contribution to economic thought, as to theology, moral philosophy, and politics, lies in his emphasis on ratiocination on the Greek ideal of accepting nothing unless good reasons can be given for it.