The Divided Self: An Existential Study in Sanity and Madness - Penguin Modern Classics by R.D. Laing

First published in 1960, this watershed work aimed to make madness comprehensible, and in doing so revolutionized the way we perceive mental illness. Using case studies of patients he had worked with, psychiatrist R. D. Laing argued that psychosis is not a medical condition but an outcome of the 'divided self', or the tension between the two personas within us: one our authentic, private identity, and the other the false, 'sane' self that we present to the world.