It is the traditional teaching of the Roman Catholic church that the conscience is the final judge whether an action is in conformity with objective law or not.
According to Thomas Aquinas, conscience is connected to the rational faculty of man. Now, what if this rational faculty is corrupted? Remember, religious, philosophical and political beliefs, misguided idealism, malicious propaganda and poor education can corrupt a person’s rational faculty.
What happens then? Well, then conscience becomes unreliable. That is why Aquinas becomes the victim of his own theory when he defended the evils of the Inquisition “in good conscience.”
Thomas Aquinas was a Dominican who was greatly influenced by the philosophy of Aristotle. Aristotle believes that human are by nature good. Benedict de Spinoza (1634-1677) also believed that men are not conditioned to live by reason alone, but by instinct. Greatly influenced by Spinoza, Giambattista Vico (1688-1744) believed that God’s law were immanent not transcendent. God places these laws in us by instinct. Immanuel Kant (1724-1804) and Gottfried Herder (1744-1803) both believed that men are guided by natural law, but unlike Aquinas, they believed that reason (not conscience) is the sole arbiter of truth and falsehood, of good and evil. These ethical theories influenced Thomas Jefferson’s “inalienable rights” which were stated in the United States Declaration of Independence.
However, some believed that nature could not provide the norm and pattern for moral behavior. The Lisbon earthquake of 1747 brought out a moral dilemma regarding natural moral law. Voltaire (1694-1778) asked if nature is good, then there must be no evil. John Stuart Mill suggested that ethical naturalism is blind to the obvious darker side of nature, the side marked by physical disorder and calamities, the aberration of the human heart and the tragedy of history. Thomas Hobbes (1588-1679) stated that the natural condition of man is war. Since the main law of nature is self-preservation, it follows that men are obliged to confer all their power and strength upon one man or upon one assembly of men that will reduce all their wills to one will. There must be someone who has the power to enforce contracts and obligation.
Going through Hobbes, John Locke, (1632-1704), like Hobbes, believe that man by nature are equal (not good). But unlike Hobbes, he believe that civil society will prosper with reason. Therefore, instead of conscience, society must set up a known authority to which everyone may appeal and obey. However, this authority should be judge in each own case. Common good is now not based in instinct and nature, but is determined by standing laws, statutes that all are aware of and agreed to.
Natural moral law is definitely a double-edged sword. If a Christian would insist that morality in embedded in nature, what do I have to lose? If that is true, then we don’t need a God to discover morality. If Darwin was right about morality, that it (like cooperation and altruism) evolved to humans (through natural selection) then who needs God.
Speaking of morality, I prefer Mr. David Ramsay Steele’s explanation regarding this issue. His view is that the structure of moral theory is just as objective as the structure of, say, medical theory. Now, practicing morality, like practicing medicine, requires an input of subjective values. In the case of morality, these values derive from empathy from other conscious beings. This empathy is in fact, almost but not quite universal among humans. (Atheism Explained – From Folly to Philosophy p.289).