Saint Thomas Aquinas, like Aristotle, regards virtue as a disposition. However, he differs from Aristotle in his emphasis on the possibility of correcting a faulty disposition by means of reason or, failing that, instruction by a reliable guide. In other words, he thinks that virtue can be taught and that it can also be strengthened by practice.

It is virtue that ought to provide the connection between natural law and human law. Natural law derives from God and, at its most basic, stipulates that "good is to be pursued and evil avoided." God decrees that certain acts are wrong at all times and in all places. It is easy to think of examples which practically everyone would agree are wrong: murder, genocide, and other crimes against humanity. However, Aquinas is concerned to ground these principles in eternal and immutable divine command rather than human consensus.

Even human law is not derived from agreement among humans but from a proper understanding and application of natural law. If people are virtuous, they will understand and obey natural law, so that the laws they make will reflect natural law. Aquinas therefore regards legislation as a science which involves applying the fixed principles of natural law to human life in order to create just human laws based on divine command. Only the virtuous will be able to do this.