The Gentlemen's Book of Etiquette and Manual of Politeness / Being a Complete Guide for a Gentleman's Conduct in all / his Relations Towards Society

In fact, the mind must be treated like the body, and however great its appetite, it will soon fall into bad health if it gorges, but does not ruminate.

Even if convinced that your opponent is utterly wrong, yield gracefully, decline further discussion, or dextrously turn the conversation, but do not obstinately defend your own opinion until you become angry, or more excited than is becoming to a gentleman.

A man in a passion ceases to be a gentleman.

One of the first rules for a guide in polite conversation, is to avoid political or religious discussions in general society.