Sentence:  Missionary outreach by friendly contact looks somewhat different when one reflects on what is known about friendly contact in metropolitan neighborhoods ; the majority of such contacts are with people of similar social and economic position ; association by level of achievement is the dominant <WSD>principle</WSD> of informal relations . 
Meaning of the word marked with <WSD> tags:  a basic generalization that is accepted as true and that can be used as a basis for reasoning or conduct