In the West, existence had largely configured itself on the Christian reading of the Bible. The Second Table of the Decalogue was taken for granted as a law also by those many happy souls who never wonder where it came from. To them it was like summer and winter, simply what there is.
In fact all these happy people lucked onto a perfect undestanding what natural law would be if it existed, but unfortunately...
This precisely is the theme of the basic controversy in the philosophy of law: Is there any natural right? The answer which prevailed prior to Socrates seems to have been negative:
In God's view, all things are fair [noble] and good and just, but men have made the supposition that some things are just and others are unjust. --
Heraclitus quoted by Leo Strauss in "Natural Right - a History".
For instance a new gun law comes out. Aren´t there many people who will say that the new law is not just?
They believe that they have their own sense or measuring stick of what is right or wrong.
Unlike Aquinas, they cannot wonder about the ancestry of their views.
Aquinas thought that the natural law is in our conscience as part of the eternal law governing God`s creation.
Aquinas in Catholic Theology quoted by Cardinal Newman :
"The natural law is an impression of the Divine Light in us, a participation of the eternal law in the rational creature" (Gousset, Theol. Moral., t. i. pp. 24, etc.).
Aquinas in his "Summa":
Aquinas learnt from Maimonides who had access to some Greek sources through the Arab language. Maimonides, who was Jewish, wrote in Arabic. When you first see it, you cannot believe your eyes: the Catholics got their Latin originals from Jewish translations of Moslem Arab documents based on Pagan Greek text.
It is a tongue twister. Once you are over your surprise, some bit of sadness sets in seeing that for centuries things like that remain known only sparcely and only to very few people and remain buried under tons of things to read.
A.N. Whitehead quoted by Leo Strauss in "Natural Right - a History".
Notice he says "the cosmic order may be thought to be", because we don't know what it really is; we only know what can be thought about it.
But that is part of another problem.
The Big Names
Heraclitus .. circa 500 BC
Socrates ... 470-399 BC
Plato ... 428-347 BC
Aristotle .... 384-322 BC
Maimonides .... 1135-1204
Aquino ... 1225-1274
Kant ... 1724-1804