His _Observations on Man: his Frame, his Duties, and his Expectations_, appeared in 1748. The principal interest in the book consists in the fact of its containing the germs of that school of moral philosophy of which Paley, Bentham, and Mill have been the most able expositors. He had imbibed the teaching of Locke upon the origin of ideas, which that first of English metaphysicians founded in Sensation and Reflection or Association, in contradiction to the old theory of _Innateness_. Although now universally received, it is hardly necessary to remark that at its first promulgation it met with as great opposition as all rational ideas experience long after their first introduction; and Locke’s controversy with the Bishop of Worcester is matter of history. It has already been stated that David Hartley was the friend of Dr. Cheyne, whom he attended in his last illness, and he numbered amongst his acquaintances some of the most eminent personages of the day. His character appears to have been singularly amiable and disinterested. His theology is, for the most part, of unsuspected orthodoxy. The following sentences reveal the bias of his mind in the matter of _kreophagy_:-- “With respect to animal diet, let it be considered that taking away the lives of [other] animals in order to convert them into food, _does great violence to the principles of benevolence and compassion_. This appears from the frequent hard-heartedness and cruelty found among those persons whose occupations engage them in destroying animal life, as well as from the uneasiness which others feel in beholding the butchery of [the lower] animals. It is most evident, in respect to the larger animals and those with whom we have a familiar intercourse--such as Oxen, Sheep, and domestic Fowls, &c.--so as to distinguish, love, and compassionate individuals. They resemble us greatly in the make of the body in general, and in that of the particular organs of circulation, respiration, digestion, &c.; also in the formation of their intellects, memories, and passions, and in the signs of distress, fear, pain, and death. They often, likewise, win our affections by the marks of peculiar sagacity, by their instincts, helplessness, innocence, nascent benevolence, &c., &c., and, if there be any glimmering of hope of an hereafter for them--if they should prove to be our _brethren and sisters_ in this higher sense, in immortality as well as mortality--in the permanent principle of our minds as well as in the frail dust of our bodies--this ought to be still further reason for tenderness for them.