The term “the inner game” was first coined by Timothy Gallwey in his seminal work The Inner Game of Tennis: The Classic Guide to the Mental Side of Peak Performance.
In this book, Gallwey discusses the difference between the “inner game” and the “outer game”. Where the “outer game” includes the techniques and skills needed to be a good player, the “inner game” is about having the right mental approach while practicing and playing.
Gallway went on to publish several more books, extending the concept of “inner game” to include books on “Golf”, “Work”, “Skiing” and “Stress”. Check out Timothy Gallway’s bibliography for more details.