To reverse the recommendation from the Sound of Music, when it comes to moral theology, the end turns out to be a very good place to start. That end, as Romanus Cessario O.P. explains in his Introduction to Moral Theology, is God. This final beatitude, in which we see God in heaven, is the fulfillment of human happiness. As Augustine famously explained: “You have made us for yourself, O Lord, and our hearts are restless until they rest in thee.” Happiness, then, is the subject of moral theology.
God's creations, including human beings, possess a nature which determines the happiness of that creature. Just as a duck fulfills its vocation by virtue of being a duck, there are patterns for people which make us the happy humans God intended us to be. Contrarily, there are behaviors, i.e. sins, which cause us to fall short of human happiness. These dictates form what is known as the natural law.
Moderns recoil at the idea of the natural law, which seems arbitrary and restrictive. Yet if the ramifications still rankle, we ought to recognize the principle. A ball is by nature spherical; it rolls and bounces. One which is irregular in shape and fails to roll or bounce is not a “good” ball. That human beings may choose to behave contrary to the natural law is indicative of our ability to choose good and evil; it does not disprove the notion altogether.
Fr. Cessario helpfully delineates “the three distinct areas of human well-being” to which that law inclines us as human beings:
“Self-preservation, the good which man has in common with all creatures: to preserve the substantial being which is possessed;
Procreation and the rearing of children, the good of the nature man has in common with sentient, but irrational, creatures: to preserve through the coupling of male and female the human species;
Knowledge of truth about God and society, the good of the nature man has in common with all intelligent beings: to act in accord with reason and to realize the potential which reason affords.”
I note in passing that only the last of these commands is enjoined about humans alone; the much-maligned second injunction, from which the Church's “controversial” proscription on contraception stems, is enjoined upon all creatures by virtue of our sentient nature. For more on this topic, see John S. Grabowski's Sex and Virtue: An Introduction to Sexual Ethics, the second volume in this series on Catholic Moral Thought.
Philosophy, and especially ethics, has been struggling to free itself from the legacy of Hume and Kant, what Fr. Cessario refers to as their “anti-metaphysical presuppositions.” For “[David Hume] eschews what he considers the groundless view that moral oughts can be discovered on the basis of an analysis of what human nature “is”, or deduced from distinctively human properties.” By “freeing” man from the dictates of human nature, man is encouraged to self-actualize, but without an end to which he can aspire, this becomes an exercise in absurdity. It also renders discussions about ethics fruitless as any behavior can be defined as ethical if the criteria are subjectively set by the acting person.
The author provides assistance in freeing the modern from the trap set by the aforementioned philosophers. He observes: “[B]ecause human reason is able to discover what suits the in-built entelechies of human nature, the Christian moral theologian can confidently expound on the teleological dimension of the moral life without undue appeal to legal sanctions and punishments.” The prose can be challenging, but the instruction is sound.
I have chosen to focus on what I believe to be the most compelling section of the book, but there is more to moral theology than the natural law. For instance, Fr. Cessario discusses the role played by virtues in creating a “pattern of a graced life” He also emphasizes the instrumentality of the Holy Spirit. Still, this introduction is probably most valuable for the cogent explanation and defense of the natural law.