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 Supremacy of the Moral Sentiments and Intellect," Combe goes on to argue that the moral sentiments are superior to those that men share with animals, and that the faculties "are right only when directed by "enlightened" intellect and moral sentiment..."they must be illuminated by knowledge of science and of moral and religious duty,"" or else the result is evil. A delicate balance of the expression of the faculties is required for man's happiness. In Section five, "The Faculties of Man Compared with External Objects," Combe identifies specific faculties and their effects on the eternal world. For example, with Philoprogenitiveness, the