Natural is an ambiguous word: the state of nature, red in tooth and claw, can seem, in the words of Thomas Hobbes, "nasty, brutish and short".
So what's so natural about the natural law?
As conceived by the Greeks, everything has a purpose or telos. The purpose of a knife is to cut, and the supreme good for a knife is to cut excellently. I recommend a ceramic knife for this purpose - it really is excellent. Natural here means something like "best expressing its proper purpose".
This raises the question - what is the proper purpose of human beings? To Aristotle and Aquinas it is to reason well, "right reason in agreement with nature" says Aristotle or "sharing in the eternal law by intelligent creatures is what we call the natural law," said Aquinas. The Catholic Church puts it this way:
So humans have been given (by God if you are deist or evolution if not) innately the knowledge of first principles or the synderesis rule. It is inscribed or written into our very nature. We know instinctively what it is to "do good and avoid evil" - these first principles are what build the excellent fulfilled life.
My son has just finished his gap year in Gambia. Whilst out swimming he was caught in a rip tide and almost drowned. A stranger paddled out on a surf board to save him - innately as it were responding in order to preserve life. Maybe you've heard similar stories of "non-reciprocal altruism" or doing good with no hope of reward.
These are general, observable ends which rational human beings pursue - to preserve life, reproduce, pursue wisdom and truth, order our social world and worship God. These are absolute, unchanging principles that arise from our understanding of human nature and what we need to flourish and grow. There is nothing very mysterious about this - just look around you and you will see these things confirmed a posteriori (meaning "by experience") by our own eyes and ears.
The ultimate end is God if you're Aquinas, or the flourishing life (Greek eudaimonia) if you're Aristotle - Aquinas believes we need to order our lives towards God by aligning our own behaviour according to the rules suggested by the natural law . These secondary precepts may change according to our situation and our perception of what we need in order to flourish and be like God.
But just how absolute are the primary precepts? In the encyclical quoted above the Catholic Church has dropped worship of God as a natural tendency and replaced it with "contemplation of beauty". Of course both God and beauty belong to the numinous realm - they inspire awe in us, and perhaps this pragmatic change recognises that the supposed God-instinct can surface in surprising ways - in great art, literature, poetry or music, or simply by gazing at the starry heavens and feeling the awesome nature of the universe.
After all, you don't have to believe in God to be a natural law theorist - just in the teleological (purposeful) nature of the way the world appears to be set up.
Click here for my introductory handout on Aquinas and natural law - the best on the net!