In a bull market, everyone looks like a genius. Warren Buffett, having just booked what he believed was the largest one-year gain any American business had ever made, refused to take the bow. He told his shareholders a big part of it was pure luck. And then spent the rest of the letter teaching the single skill that matters most to an investor: How to tell whether a result came from ability or from a rising tide. The answer separates the managers you should trust from the ones you should flee, and the businesses that last from the ones that will die.