When the season is over, the Chicago Cubs' ace right-hander Larry Jackson goes home to Boise, Idaho, to spend the winter, and he usually has gone a disappointed man. Jackson has been in the big leagues for nine seasons and has been considered one of the National League's best pitchers for most of them, but as 1964 began he had yet to do what every pitcher dreams of doing—win 20 games in one year. So, with little else to do in Boise, Jackson sat down last year and started persuading himself that he could be a 20-game winner. "I told myself I could win 20 games all last winter. I kept it up during the spring and then all through the season," said Jackson last week. He may turn out to be the best example of the value of positive thinking since Norman Vincent Peale proposed the theory. In the first year of Jackson's experiment he has won his 20 games. Tuesday he pitched a six-hit, 6-1 victory over the pennant-contending Cincinnati Reds and became the major leagues' first 20-game winner of 1964. But his new self-inspired confidence propelled him further. Four days later he threw another six-hitter, defeating Milwaukee 5-3, for his 21st win and a big edge toward becoming baseball's winningest pitcher of the year. And what does Jackson think of it all? "I am no better this year than before [his ERA is, in fact, higher than in 1963], but I kept telling myself that I had to average four wins a month to make 20, and that's just what I did."