To begin I will talk about "Natural Law" (ius naturale), these are laws that are not written down but were considered "a given". These laws were considered common to all nations, and were upheld by the "instinct of nature" (Etymologies, V.iv).
A selection of some of the laws considered natural are as follows; the union of man and woman, a child's inheritance and education, "the common possession of everything", our right to a "single freedom", the right to acquire whatever is taken from the earth, sky or sea. These are very telling statements, we get some idea of how "natural" isn't really natural at all; in reality these philosophers were imposing an order of their own creation upon the world.
While the so called "sexy times" between a man and a woman is biologically natural enough the rest of these statements are societal constructs; because we are the dominant and superior species created by a god (who looks suspiciously like us apparently) we have the natural right to claim what we like from the earth, sky and sea. The "single freedom" clearly did not refer to freedom from slavery, as this was fairly rampant all over the place, so I am not quite sure what that means - if I was to hazard a guess I would say this "single freedom" is most probably our ability to decide our fate (speaking from a Catholic perspective of course) we alone are all blessed with the ability to discern the true nature of things and therefore free to choose the right courses of action.
It is important to remember this construct of a "natural order" to things; I have used the concept of natural laws here because they highlight more readily how the scholars and philosophers of the time would super-impose order to otherwise arbitrary systems; freedom is a human construct so there is nothing natural about it, and this "right to acquire" is arrogant in the extreme -but its not like we got over that is it, "manifest destiny" anyone?
All in all, the very human desire to make the world an understandable place with ordered and recognizable systems may have lead to some incorrect assumptions, but without it? Hell - I doubt we would have gotten this far.
Just remember that it is "natural" and right to educate a child - so paying for education is incorrect! (This moral message is brought to you by "The Ancients"!).
Next up: Time and its calculation.