First published in 1974, the book approaches tennis through attention, learning and self-judgement. Gallwey's distinction between Self 1 and Self 2 gives readers a way to discuss the difference between the voice issuing instructions and the capacities involved in actually performing and learning a skill.
Those labels work as a coaching model, not as a literal anatomical division. Their usefulness lies in making a common experience easier to notice: the player who responds to one mistake by adding so many instructions that the next attempt becomes more difficult.
