Question: "What is natural law?"
Answer: When Thomas Jefferson wrote, "We hold these truths to be self-evident…" he was referring to natural law. Natural law is the universal standard that directly reflects human nature; natural law can be determined by careful consideration of the human condition, regardless of cultural influences. Jefferson considered the equality of man, and life, liberty, and the pursuit of happiness (purpose and livelihood) to be born directly from the nature of humanity.
The concept of natural law has evolved and will continue to do so. Plato hinted at it when he wrote of the ultimate, perfect forms that nature attempts to reflect. Aristotle believed there was a common law that applied to all of nature, and governments would do well to attempt to live by it, even if they had to resort to not governing at all. The Stoics taught that the universe was ruled by a divine or eternal law, and "natural law" was mankind's guidance for living according to that divine law.
Cicero believed natural law comes directly from God. He defined it as "the safety of citizens, the preservation of states, and the tranquility and happiness of human life." Natural law supported the health and well-being of society because it was only in a healthy, peaceful society that individuals could achieve "happiness"—contentment and purpose. Cicero's definitions influenced the legal system of the Roman Empire and the American Revolution, with its belief that even the monarchy of Great Britain were subject to whatever law profited the kingdom as a whole. Thomas Hobbes' interpretation was not so civic-minded. He believed natural law was more individual and based on personal survival and prosperity. The primary purpose of society is to avert war, Hobbes said, because war harms individuals.
If developed properly, civil law (also known as "positive law") is derived from natural law. Where natural law is vague (citizens should be safe), governments must develop more specific standards (violent criminals will be prosecuted). In an ideal world, everyone would be internally ruled by natural law. Government would be all but unnecessary, and all humanity would be willingly subject to universal standards.
The problem, obviously, is that mankind is unable to agree on the definition of natural law and has no hope of agreeing on how it should be enforced. Hobbes said the fulfillment of natural law was protection of the individual, Cicero said it was support of the state, and Jefferson said it was life and liberty—despite the fact that he owned slaves.
In truth, natural law is given by Him who created nature, and most philosophers have looked to God for the definition. The Bible does support the idea of natural law, but not in the way most think.
Paul spoke of natural law in Romans 2:14-15: "For when Gentiles who do not have the Law do instinctively the things of the Law, these, not having the Law, are a law to themselves, in that they show the work of the Law written in their hearts, their conscience bearing witness and their thoughts alternately accusing or else defending them." God made His law evident in the hearts of all mankind. But, because we live in a fallen world with a sin nature, we are incapable of completely knowing what God's law is, and we cannot follow it (Romans 7:14-25). Therefore, God gave us His revealed law, inspiring the prophets and the writers of the Bible to explain how to live according to the natural law that we catch glimpses of, but can never really grasp.
The natural law God gave to humanity is fairly similar to what most cultures would include in their mores: procreate (Genesis 1:28) and respect life (Genesis 9:5-6). But we are more than biological life forms. As "new creations," made in the image of God, we understand that God's law isn't life, liberty, or the pursuit of happiness. And it's not the safety of citizens, the perseveration of states, or peace. It is this: love God, love others (Matthew 22:37-40). If that love entails personal harm (Matthew 5:27-30), removal of contentment (Matthew 5:39-42), dissension in the family (Matthew 19:29), or even loss of life (Matthew 10:39), we are to welcome it. Our spiritual nature is greater than our physical nature, and our spiritual natural law trumps even physical life.