Mark Twain, from "What Paul Bourget Thinks of Us," North American Review (1895), a response to Bourget's Outre Mer:
I think M. Bourget meant to suggest that he expected to find the great "American soul" secreted behind the ostentations of Newport; and that he was going to get it out and examine it, and generalize it, and psychologize it, and make it reveal to him its hidden vast mystery: "the nature of the people" of the United States of America. We have been accused of being a nation addicted to inventing wild schemes. I trust that we shall be allowed to retire to second place now.
There isn't a single human characteristic that can be safely labeled "American." There isn't a single human ambition, or religious trend, or drift of thought, or peculiarity of education, or code of principles, or breed of folly, or style of conversation, or preference for a particular subject for discussion, or form of legs or trunk or head or face or expression or complexion, or gait, or dress, or manners, or disposition, or any other human detail, inside or outside, that can rationally be generalized as "American."
Whenever you have found what seems to be an "American" peculiarity, you have only to cross a frontier or two, or go down or up in the social scale, and you perceive that it has disappeared. And you can cross the Atlantic and find it again. There may be a Newport religious drift, or sporting drift, or conversational style or complexion, or cut of face, but there are entire empires in America, north, south, east, and west, where you could not find your duplicates. It is the same with everything else which one might propose to call "American."
(Later in the essay, he thinks he's found a defining characteristic: Americans universally like ice water, and Europeans don't.)