This from the resemblance of the external actions of animals to those we ourselves perform, that we judge their internal likewise to resemble ours; and the same principle of reasoning, carry'd one step further, will make us conclude that since our internal actions resemble each other, the causes, from which they are derive'd, must also be resembling. When any hypothesis, therefore, is advance'd to explain a mental operation, which is common to men and beasts, we must apply the same hypothesis to both.

A Treatise of Human Nature