“A foreigner is a man who laughs at everything except jokes. He is perfectly entitled to laugh at anything, so long as he realises, in a reverent and religious spirit, that he himself is laughable.”

"Fads and Public Opinion" http://www.online-literature.com/chesterton/what-i-saw-in-america/10/
What I Saw in America (1922)
Context: A foreigner is a man who laughs at everything except jokes. He is perfectly entitled to laugh at anything, so long as he realises, in a reverent and religious spirit, that he himself is laughable. I was a foreigner in America; and I can truly claim that the sense of my own laughable position never left me. But when the native and the foreigner have finished with seeing the fun of each other in things that are meant to be serious, they both approach the far more delicate and dangerous ground of things that are meant to be funny. The sense of humour is generally very national; perhaps that is why the internationalists are so careful to purge themselves of it. I had occasion during the war to consider the rights and wrongs of certain differences alleged to have arisen between the English and American soldiers at the front. And, rightly or wrongly, I came to the conclusion that they arose from the failure to understand when a foreigner is serious and when he is humorous. And it is in the very nature of the best sort of joke to be the worst sort of insult if it is not taken as a joke.