I just came across a pretty interesting investment story, and the experience of Japan’s stock-market legend, Kawaginzo, made me think for a long time. This guy went from being destitute before the age of 30 to rolling a 70-yen principal into a fortune of 20 billion yen—one of the key things being that he could flawlessly escape the top when the stock market went crazy. But in the end, because he “grabbed one more mouthful,” he lost everything—300 million,000,000 yen. The contrast here is really worth reflecting on.

Speaking of his legend, it all starts in 1931. Back then, after spending three years studying economic books at the Osaka Library, Kawа officially entered the market with 70 yen that his wife borrowed. He had a habit: every day he collected data, called securities firms, and pushed market research to the extreme. By the end of World War II, he had already seen that sheet metal would rise in price; sure enough, the public bought huge quantities to build temporary housing, causing sheet metal prices to soar by dozens of times.

In the 1970s, Japan’s stock-market legend saw that after the oil crisis, the government would stimulate the economy through infrastructure. So he went on a big buying spree of Japanese cement company stock that had dropped to over 100 yen. Three years later, that investment made him 30 billion yen. In the 1980s, he got even more impressive: he found that the Hishikari ore vein in Sumitomo Metal Mining had been seriously undervalued. After quietly setting up his position, the stock price skyrocketed to more than nine times the buy-in price—and he pocketed another 20 billion yen.

But the most remarkable thing about this Japan’s stock-market legend isn’t really his stock-picking—it’s his ability to escape the top. His investment in Sumitomo Metal Mining is a typical example. When the market went into a frenzy and pushed stocks higher, he did the opposite and quickly sold off. As a result, just three weeks later, the stock price crashed to one-third of his selling price. He summarized this logic into the “Eight-Tenths Full” philosophy: selling stocks is like eating—only eating to 80% fullness is wisdom. What’s hardest to grasp in the market isn’t the timing to buy—it’s the timing to sell. Greed is often what causes things to go wrong right here.

He also created the “Turtle Three Principles”—to dig for overlooked potential stocks and hold them long-term, to study the market personally every day, and to never be overly optimistic. He never believed in “good news” from newspapers, because by the time such news is reported, the stock price is usually already close to its peak.

That said, it’s also necessary to mention his blood-and-tears lesson. In the late 1970s, he judged that the Soviet invasion of Afghanistan would push up the prices of non-ferrous metals, so he went on a large buying spree of related stocks. But this time, unusually, he lost his composure. Driven by greed, he insisted on not selling, and in the end he missed the selling point. Seeing his profit of 30 billion yen turn into nothing but “paper wealth,” this crushing defeat formed a cruel contrast with the “Eight-Tenths Full” wisdom he had championed early on.

So, the hardest thing to control on the road to investing has never been knowledge or experience—it’s that beast called “greed” inside human nature. The answer Japan’s stock-market legend gave with his legendary life is eight words: invest rationally, exit calmly. With this ruler that measures wealth and risk, have you got it firmly in your hand?