How to Read Human Nature is a practical manual in the popular character-reading tradition of the early twentieth century. Atkinson begins by defining his subject. Human Nature, as he uses the phrase, is the particular study of an individual's character, disposition, and temperament, a concrete branch of psychology concerned with how people actually appear and behave rather than with abstract laws.
His central principle is a correspondence between the inner and the outer. A state of mind produces an appropriate physical expression, and a settled mental trait, repeated often enough, becomes fixed in the face, the gait, the tone of voice, and the manner. He cites William James, Herbert Spencer, and other writers of the New Psychology to argue that this action runs both ways: assuming the outward posture of an emotion can even call up the feeling itself.
From this principle Atkinson builds a two-part scheme. Character is the inner phase, the sum of qualities a person inherits from the long line of ancestors and absorbs from environment. Personality is the outer phase, which he traces to the Latin for an actor's mask, the form through which the hidden character sounds and shows itself to others. The whole art of reading people rests on the link between the two.
Much of the book is a catalog. He classifies people by Quality, a felt degree of fineness or coarseness, and by three temperaments, the Vital, the Motive, and the Mental, each with its own build, tastes, and tendencies, with the balanced type held up as the most capable. He then runs through the mental qualities in the language of the old phrenologists, before turning to physiognomy: faces, chins, mouths, eyes, noses, and miscellaneous signs such as the voice, the laugh, and the handshake.
The method is meant to be worked, not just read. Atkinson tells the student to learn the inner qualities first, then the outer marks that accompany each, and to test the system on people whose character is already known. Once the principle is grasped, he says, the rest is practice. Read today, the book is a period document: it leans on phrenology and temperament theory that later science set aside, yet its core intuition, that conduct and bearing reveal the person, remains its lasting appeal.