Human uses of mammals that "one is well advised to study the behaviour rather than attempting to get at any underlying emotion". Charles Darwin however had accepted the idea of emotion in animals, writing his 1872 book "The Expression of the Emotions in Man and Animals" on the subject. Darwin believed that mammals had social, mental and moral lives. In "The Descent of Man" (1871), he wrote: "There is no fundamental difference between man and the higher mammals in their mental faculties." Biologists remained wary of the idea, even for the great apes, but this led to serious difficulties, as Donald O. Hebb explained
The Constitution of Man The Constitution of Man The Constitution of Man (or more completely, The Constitution of Man Considered in Relation to External Objects) first published in 1828 is a work by George Combe, who is credited with popularizing the science of Phrenology. Combe argues that the human mind is best understood through Phrenology, and that the relative size of the various regions of the brain defined by Phrenology determines a persons behavior and potential interactions with the external world. In "The Constitution of Man" Combe uses Phrenology to create a practical science of morality, proposing that conforming to Natural Laws leads to