That's part of the problem.
The Founders believed that certain laws affected all nations equally and these laws were not within the power of man or man's government to change.
There is some contention as to if a particular version was used or several were used to come to a consensus, but most generic name for these laws is the Law of Nations.
Are you familiar with the Law of Nations concept?
I guess so. I've heard the term and know it's in the Constitution, and I've certainly heard of the book by that name.