Recently, in a discussion with some friends, one of them said the idea of the natural law was an invention of mediaeval Catholic theologians. Is this true?
It is most certainly not true.
The concept of the natural law predates Christianity by many centuries.
For the benefit of those unfamiliar with the natural law, it will be helpful if I first explain briefly what it is.
In simple terms, the natural law is the series of rights and duties, or rights and wrongs, based on human nature.
That is, merely by considering human nature we can deduce that certain acts are in keeping with that nature and contribute to human flourishing and are therefore morally correct, while other acts are contrary to that nature and are morally wrong.
Among the latter are killing the innocent, stealing and lying.
These are wrong not because the Bible says so, but because they are contrary to human nature.
Or, if you like, the Bible says they are wrong because they are wrong in themselves.
If we ask what human nature is, it is simply what makes a particular being human rather than something else like a tree or a dog.
In short, it is what all human beings have in common. We all have human nature, just as dogs have dog nature and tables have table nature.
Based on human nature are not only moral precepts about right and wrong but also human rights.
For the very fact of being human, a person has such basic rights as the right to life, to liberty, to follow his or her conscience in matters of religion, to marry and form a family, etc.
Many of these human rights are listed in the United Nations Universal Declaration of Human Rights of 1948.
To believe in human rights is therefore to believe in the natural law.
Another concept based on the natural law is crimes against humanity.
Even though leaders of some nations authorised practices like genocide, use of chemical weapons against their own people or lethal experiments on the handicapped, prisoners and the insane, they were still accused of crimes against humanity, because human rights take precedence over manmade laws and decisions.
The Nuremburg trials after World War II judged matters such as these and found the perpetrators guilty.
The fact that the accused had acted according to the laws of their country was not a legitimate defence.
The higher natural law and the human rights based on it take precedence.
Getting back to your question, belief in a law based on human nature goes back long before Christ, let alone before mediaeval theologians.
There is evidence of belief in a universal law binding on all human beings in Oriental literature, especially in China, many centuries before Christ.
But a more developed concept of the natural law is found in the ancient Greeks, in writers like Sophocles (c 497-406 BC), whose play Antigone features a clear understanding of a natural law which is higher than the law of the king. The play was first performed in 441 BC.
Other ancient Greeks who presented a concept of a natural law that is universal and known to all were Thucydides (c 460-400 BC) and Zenophon (c 427-355 BC).
In the 4th century BC, a great exponent of the natural law was the Greek philosopher Aristotle (384-322 BC).
In his Nicomachean Ethics, he distinguished clearly between what is just by nature and what is just by human law (Book V).
And, in his work On Rhetoric, he wrote that “there is in nature a common principle of the just and unjust that all people in some way divine, even if they have no association or commerce with each other” (Book I, Ch 13).
One of the greatest exponents of the natural law in ancient times was the Roman jurist and philosopher Cicero (106-43 BC).
In his work Laws, he wrote: “What is right and true is also eternal, and does not begin or end with written statutes… From this point of view it can be readily understood that those who formulated wicked and unjust statutes for nations, thereby breaking their promises and agreements, put into effect anything but ‘laws’. It may thus be clear that in the very definition of the term ‘law’ there inheres the idea and principle of choosing what is just and true… therefore Law is the distinction between things just and unjust, made in agreement with that primal and most ancient of all things, Nature, and in conformity to nature’s standard are framed those human laws which inflict punishment upon the wicked but defend and protect the good.”
Later, Christian theologians like St Augustine and St Thomas Aquinas based their teaching on the natural law on these ancient writers. So the concept of the natural law is ancient indeed.