There are essentially four ways we come to know things: observation, reason, feeling, and authority.
Observation is the source of most of our scientific knowledge. We, for example, observe that water boils at 212 degrees and freezes at 32 degrees. We can’t figure these things out using abstract reason; these are things we observe and test. And observation teaches us a lot! For example, the scientific method is grounded in observation.
Reason is the source of much of our mathematical knowledge. Consider the Pythagorean Theorem. On a right triangle A squared plus B squared equals C squared. This is always true. We don’t need to go around and measure every triangle to know this; this is something that was discovered by reason and continually confirm by reason.
Feeling also reveals truth, though the truths that they reveal are often personal. I prefer chocolate to strawberry ice-cream. This is a personal preference based on my sense of taste. I know I have a headache when my head hurts. These are truths, but unlike the first two types of truth, they are personal and changeable.
Finally, the majority of what we know is grounded in authority. I have never been to Singapore, yet I believe there is such a place as Singapore based on the testimony of others. Similarly, I believe a man named Napoleon lived approximately two centuries ago based on the authority of those who knew him. And while I can’t prove that my parents are really my parents based on abstract reasoning, I trust them and take their word for it.
If, as Christians believe, ethics are natural, if they have some sort of transcendent existence beyond the opinions and preferences of men, then how do we know them?
During the Enlightenment philosophers believed that observation alone (empirical knowledge) could explain everything. The scientific method advanced technology rapidly and they believed they could use it to find answers to difficult political and moral questions. They failed. Observation shows us what is, but it cannot tell us what ought to be. We can observe that this or that action will lead to pain, but observation alone cannot tell us whether we ought to do or avoid things that cause pain. Something more, some sort or assumed moral standard, is needed.
Reason has similar limitations. Reason can help us figure out the steps we need to take to get to our goals, but it cannot tell us which goals we should have. For example, reason may be able to help us invent a better missile, but it cannot tell us when we ought to launch or refuse to launch a missile. Reason *may* be able to tell us how we can be more happy, but reason itself cannot tell us whether or not happiness should be the goal of human life. In short, reason is instrumental—it can help us get from point A to point B, but it cannot tell us whether point B is worth pursuing.
Feeling can tell us things about ourselves, but they cannot help us lay out rules to govern the behavior of others. For example, a factory owner may feel just fine about underpaying his employees, but his employees will likely have different feelings. I may feel that sexism is abhorrent, but someone raised in a chauvinistic culture may have very different feelings.
Observation, reason, and feelings all fail to provide a sound basis for ethics. This is why God gave us the Bible. Observation, reason, and our feelings all have value, but none of them can give people moral certainty. And yet moral certainty is exactly what we need if we are going to live and behave responsibility. How should we treat others? What should a family look like? How should we treat the property of others? How should we view food, rest money, our children, and our parents? God’s revealed word gives us guidance in all of these crucial areas.
A truth that rests on the authority of another is only trustworthy to the extent that (1) the authority is knowledgeable and (2) the authority is trustworthy. I would not trust information about Singapore from someone who has neither studied nor been to Singapore. Likewise, I would not trust someone to tell me about Singapore if they had proved themselves to be deceitful about other things. God is both knowledgeable (He is omniscient) and honest (He is the Truth—John 14:6). Given this, we can fully trust the Bible which He has inspired. Though Christians disagree about how to interpret parts of the Bible, it is a standing belief among Christians that certain knowledge of ethical matters comes from the Bible alone.