Darwin, however, excelled as a theorizer, and The Expression of the Emotions is much more than a body-language dictionary. His main interest was the relation between each emotion and the form of its expression. His answer is contained in three principles governing emotional expression. Some emotions are expressed in a way that is posturally appropriate for the related behavior. His second principle, antithesis, is a more remarkable insight.
Some emotions appear to be expressed by a posture opposite to that of the opposite emotion.
As an example, Darwin used a famous pair of pictures of a dog. One shows a hostile, threatening dog, expressing its emotion by means of the serviceable associated habits. Darwin interpreted shrugging the shoulders in the same way. Shrugging the shoulders likewise expresses patience, or the absence of any intention to resist. In a set of four photographs, he shows how shoulder shrugging is posturally antithetical to someone who is in control or command.