In the mid-nineteen-seventies, the cognitive psychologist Ellen Langer noticed that elderly people who envisioned themselves as younger versions of themselves often began to feel, and even think, like they had actually become younger. Men with trouble walking quickly were playing touch football. Memories were improving and blood pressure was dropping. The mind, Langer realized, could have a strong effect on the body. That realization led her to study the Buddhist principle of mindfulness, or awareness, which she characterizes as “a heightened state of involvement and wakefulness.