There's a nice piece by Reeves Wiedeman over at Buzzfeed on Timothy Gallaway's 1972 book, The Inner Game of Tennis:

The Inner Game was published in an era when “sports psychology” was a phrase few had ever heard. Its release was a relative sensation, and 40 years later, now one book among many in the ever-expanding self-help section, it continues to sell thousands of copies each year. It tops Amazon’s sports psychology category, and falls behind only recent best-sellers by Jimmy Connors and Andre Agassi in the tennis category. (On the list of tennis instructional books, Brad Gilbert’sWinning Ugly is a distant second.) The trade paperback alone has sold more than 150,000 copies since its release in 1997, according to Nielsen BookScan. Though pro tennis players are ironically loathe to talk about the mental side of their game, chances are good that many of the competitors at Wimbledon have read Gallwey’s book.

And yet, despite such ubiquity, few people are able or willing to explain why they find the book so useful. Its proponents are hesitant to boil the Inner Game down to a formula in part because, the theory goes, thinking about the Inner Game defeats the very purpose of the Inner Game. When pressed, though, they offer something like this:

Performance = Potential – Interference

The upshot is that every professional athlete, and most amateur ones, already know how to perform — to properly swing a racket, shoot a basketball, or go up-and-under around an offensive tackle. The potential of an athlete with the physical gifts and technical training of a Roger Federer, a LeBron James, or even a Lawrence Jackson is practically limitless. The difference between that potential and their actual in-game performance is everything that can go wrong in the chain of communication between the brain and the body. “Performance rarely equals potential,” Gallwey says. “A little self-doubt, an erroneous assumption, the fear of failure — that’s all it takes to greatly diminish performance.”