"Iceberg theory" refers to Hemingway’s notion that stories could be made stronger by omitting key pieces of information. In Hemingway’s view, the writer should have a complete understanding of his characters and their motivations, but the story, as written, should include only the surface facts of the action (the "tip of the iceberg"). In this way, he believed, the reader could "fill in the gaps" of his understanding by paying close attention to the events that are described. The "truth" of the writing would necessarily reflect the deeper emotional or psychological qualities of the characters without having to explain them.
One example from Farewell to Arms that illustrates this principle can be found in Chapter 27:
I was always embarrassed by the words sacred, glorious, and sacrifice and the expression in vain...There were many words that you...
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