We Baptists don’t have a handy, immediate source for questions like this. We tend to prefer to start with biblical verses (such as “love your neighbor as yourself”) and then reason from there, even if the specific issue in mind isn’t addressed by the verse itself (so, since we are called to love our neighbors, that is, to do unto them as we would have them do unto us, and Jesus implies that everyone is our neighbor, then slavery cannot be morally right). This is persuasive up to a point, but on some issues, the opposing viewpoint is able to argue right back by reasoning from Scripture. (On the slavery issue, Christians in the south argued that it was justified by God’s cursing of Canaan in Genesis 9, by the fact that OT saints like Abraham had slaves, and by the fact that no one in the NT condemns it—in fact, in Philemon, Paul sends a runaway slave back to his master.) So, is there another authority we can reason from besides just the Bible in order to make our case? The tradition of Christianity says that there is: a system of ethical reasoning known as “natural law.” The basic principle is that we can look carefully at the world around us to discern God’s intent from the way things are designed, and then try to fulfill that intent. On the issue of slavery, natural law helpfully shows us that all people in the world are born the same way, develop the same way, have the same kind of moral and intellectual faculties, and (thanks to more recent science) share essentially the same DNA—thus there is no basis, from natural law, to give one race or group superiority over another: the natural world itself tells us that slavery is immoral.
Thomas Aquinas and Natural Law
The Natural Proof for the Existence of God
Aquinas wrote a number of powerful proofs for the existence of God. Unlike Anselm, whose argument rested only on the logic inherent in the idea of “God” itself, these arguments start from observations of the natural world that everyone can agree on: things like—“everything in motion had to be set in motion by an outside force,” and “everything that exists, exists because it was caused—in the natural world alone, we do not observe an effect without a cause.” Taking these observations, he devised a system which showed the absurdity of an “infinite regression.” That is to say, you can’t just have an infinite span of “caused events” in eternity past, nor an infinite series of things in motion, without having an “uncaused cause” or an “unmoved mover” somewhere back there who started it off. Logically, an infinite regression is an impossibility; otherwise we would not currently exist (because the logical idea of an infinite series before our existence means that, ultimately, that series will never actually reach the point where it produces us, or else it would not be infinite). But we do exist, therefore there must be an “uncaused cause.”
Natural and Supernatural Reasoning about Human Perfection: the Seven Virtues
Aquinas also gave us a helpful breakdown of the dynamics of human morality, the perfection of which are called “virtues.” He saw that there were some human virtues which could be deduced from reasoning about the natural world alone—these were the four “classical virtues” from Greek philosophy; and three virtues that we could not have discerned from nature, but were revealed to us in Christ.
Prudence, or wisdom—the practice of careful, thoughtful discernment about our actions and their consequences (i.e., “common sense,” which happens not to be very common, unfortunately)
Justice, or fairness – doing right to others (including honesty, give and take, keeping promises, etc.)
Temperance, or restraint (self-control) – reining in our appetites so they don’t rule over us; practicing moderation
Fortitude, or courage – the practice of confronting fear and uncertainty, and of sticking to it in hard situations
Faith – belief in God and obedience to him; more than just intellectual assent to a list of doctrines, true faith is a relational trust that should transform your life
Hope – setting our eyes on God’s ultimate victory and the triumph of his love over all other things; refraining from despair and refusing to give up
Love, or charity – Making God and others the center of our lives rather than pursuing our own self-interest; seeking the glory of God in worship and in our daily actions, and seeking the good of our neighbor in kindness and forgiveness. Not so much a “feeling” as an act of will: “Do not waste time wondering whether you ‘love’ your neighbor; act as if you did” (C. S. Lewis).