Sentence:  "Mr. Francis seems to understand the fundamental principle of all moral accounting, the great ethic rule of three. Let A do to B, as he would have B do to him; the product will give the rule of conduct required." My father smiled at this reduction of the <WSD>golden rule</WSD> to arithmetical form. 
Meaning of the word marked with <WSD> tags:  The principle that one should treat other people in the manner in which one would want to be treated by them.