For an introvert his environment is himself and can never be subject to startling or unforeseen change.

Crisp frames introversion as a kind of self-contained habitat: the introvert’s primary “environment” is the inner life rather than the shifting social world. If one’s attention and emotional investment are directed inward, external upheavals—new places, crowds, fashions, or social expectations—lose some power to shock. The line also carries an implicit contrast with the extrovert, whose sense of self is more entangled with surroundings and other people, and therefore more vulnerable to disruption. In Crisp’s hands, this is less a clinical definition than a wry, aphoristic defense of solitude as stability and self-sufficiency.