Bill Ruane, one of the most successful stock pickers of his generation. When Warren Buffett closed his investment partnership in 1969, he recommended Ruane as a replacement for himself. Until his death in 2005, Ruane’s Sequoia Fund generated stunning returns. He almost never granted interviews, but we spoke at length about the four guiding principles he had learned in the 1950s from “a major star” named Albert Hettinger. “Those simple rules have been of enormous importance to me,” said Ruane. “They formed the basis for a large part of my philosophy ever since.… And they are the best advice I can give people.” First, warned Ruane, “ Do not borrow money to buy stocks .” He recalled an early experience when, by using leverage, he “took six hundred dollars and multiplied it many times.” Then “the market cracked” and he was hit so hard that he sold out and was “back almost to square one.” As he discovered then, “You don’t act rationally when you’re investing borrowed money.” Second, “ Watch...