I think of this as being defined as the belief that certain moral rules are inherent in the nature of humanity.
Here's a more detailed description, laying out multiple schools of thought: http://www.utm.edu/research/iep/n/natlaw.htm
Here's another discussion: http://home.nvg.org/~rchg/anarchy/secF7.html
Here's an essay of that name by Lysander Spooner http://www.jim.com/spooner.htm
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