Although modern economics has much of value to say about markets, it sets aside the ethical questions raised by the market economy such as materialism or economic injustice. Theologians anxious to take up ethical concerns raised by market outcomes accordingly tend to dismiss economics, but thereby lose access to the genuine insights economists have into the workings of markets and the influence of incentives on individual behavior. This dissertation argues that the thought of St. Thomas Aquinas can help us overcome the resulting impasse. For Aquinas, economic concerns cannot be meaningfully separated from ethical concerns, but at the same time his framework can accommodate many of the genuine insights of economists.   The first two chapters of the dissertation explore the impasse between theologians and economists. The first argues that theologians encounter difficulties when they attempt to criticize markets without first developing their own systematic account of human behavior. To develop a meaningful account of economic justice requires an account of the role economic goods play in a good human life. The second defends economists' model of human behavior as rational choice against naÏve critiques, but argues that their model is not as value-neutral as they believe.   The third chapter develops Aquinas's account of human happiness, with an emphasis on its connection to his metaphysics and the implications it has for the exercise of practical reason. I demonstrate that the rational choice model would not be suitable in such a world, and that there are compelling reasons to take Aquinas's richer account of practical reason seriously. The fourth develops the argument that economic goods are properly ordered by reason, with the crucial conclusion that our desire for material goods is properly bounded. These chapters offer an account of why economists' models of human behavior are plausible, yet inadequate.  The final chapter uses Aquinas's account of private property to link his views about proper consumption to questions of economic justice. Aquinas's nuanced views on private property accommodate the insights of economists about the role of markets and incentives, while still ordering economic goods to the virtues, including the virtue of economic justice.