This is, I take it, the best known passage on Natural Law. It’s only a couple of thousand years old.
“There is indeed a true law—right reason—that is in harmony with Nature and present in all things, unchanging and eternal and that guides us to our duty by its commands and deflects us from wrongdoing by its prohibitions. Its commands and prohibitions never fail to prevail with the good but they have no power to influence the wicked. It is not right to legislate against the requirements of this law and it is not permitted to limit its application. It is impossible for it to be repealed in its entirety and we cannot be exempted from this law even by the Roman people or by the Senate. We do not need to seek out a Sextus Aelius to interpret or expound this law nor will there be one law in Rome, another in Athens, one law at one time and a different one some time later. One eternal and unchanging law will govern all peoples at all times and it will be, as it were, the single ruling and commanding god of the whole human race. That god is the creator of the law, its proclaimer and its enforcer. The man who does not obey this law is denying his own nature and, by rejecting his human nature, he will incur the greatest of punishments, even though he will have evaded the other things that are thought of as penalties.” Cicero, De Republica III xxii
And some would also point to Antigone by Sophocles. Antigone upbraids Creon, the ruler, for forbidding her to bury her brother’s body.
“But all your strength is weakness itself against
The immortal unrecorded laws of God.
They are not merely now: they were, and shall be,
Operative for ever, beyond man utterly. “
I like to compare Natural Law to a washing machine. If we expect our washing machine to do its job well and to last a long time we need to respect its nature. It should be used according to its design and manufacture. We may be able to suss this out through examination, but we will be much helped by the maker’s handbook. The only difference is that we own the washing machine, so we are free to mishandle or even destroy it, whereas we don’t own our human nature, we receive it from God.
We are fortunate in having the maker’s human handbook: we call it Scripture. And we also have an expert, appointed by God, to give us the detail. We call it the Church. But beyond that, as Cicero and Sophocles suggest, we have a capacity to judge whether our actions are consistent with human natural law, or, like Creon, to act against it.
An example of such judgment is the recognition that human beings flourish in society, so it follows that, for example, stealing is against the law of our nature. The other approach is structural: from considering our physicality we can discern which activities are appropriate and which aren’t. Not surprisingly this has a strong relevance to the uses of human sexuality. Nevertheless the instructions as laid down are not the last word. That comes from our own judgment when we have to make decisions about our own choices of action. This also comes from our human nature, central to which is reason and free will.
We may need to remember this when others criticise us for holding on to moral values in a society which, for a variety of reasons, often strays from natural law. We do not disagree because the Church says otherwise: it is us, in the sight of God, who make the final decisions and are responsible for them.