Most people recognize aphorisms like "Nothing great was ever achieved without enthusiasm," from Ralph Waldo Emerson, and its seeming opposite, "Intense feeling too often obscures the truth," from Harry Truman.

James Geary, the editor of the compendium Geary's Guide to the World's Great Aphorists, says the aphorism is the oldest and the shortest literary art form on the planet. But what qualifies as an aphorism?

Geary tells Robert Siegel that he has five laws: "It must be brief. It must be definitive. It must be personal — that's the difference between an aphorism and a proverb. It must be philosophical — that's the difference between an aphorism and a platitude, which is not philosophical," he says. "And the fifth law is it must have a twist. And that can be either a linguistic twist or a psychological twist or even a twist in logic that somehow flips the reader into a totally unexpected place."

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