WHAT IS NATURAL LAW?
A fully decidable (universal) Law of Ethics.
What do you mean by ethics?
The law of cooperation and conflict resolution.
What is this law of cooperation and conflict resolution?
Reciprocity.
WHAT IS RECIPROCITY?
In the Negative (Silver Rule, or via-negativa): The requirement to avoid the imposition of costs on that which others have born costs to obtain an interest in, without imposing costs upon that which others have likewise born costs to obtain an interest in.
In the Positive(Golden Rule, or via-positiva): the requirement that we limit our actions to productive, fully informed, warrantied, voluntary transfers, free of the imposition of costs by externality, upon that which others have obtained by the same means.
As determined by the either any change, or the total change in the inventory that all parties both internal and external to the action have born costs to obtain an interest without imposition of costs upon others directly or indirectly by externality.
—“All of ethics can be reduced to [is a subset/special application of] the degree of reciprocity & the accounting thereof.— James Augustus
WHY DOES RECIPROCITY SERVE AS NATURAL LAW?
Because it is apparently impossible to contradict reciprocity in cooperation (ethics), and as such it provides perfect decidability in all contexts of cooperation at all scales in all times, and under all conditions.