A central theme in C.S. Lewis' autobiographical book "Surprised by Joy" — and the obvious source of its title — is what he terms "Joy."

Yet Lewis struggles to convey what he means. "Longing," he calls it, sometimes using the evocative German synonym "Sehnsucht." He describes fleeting instants when, encountering a landscape or phrase or musical passage, he suddenly and unexpectedly "desired with almost sickening intensity something never to be described."