David Hartley (1705-1757)

English philosopher and physician. In Observations of Man (1749) he introduced the theory of psychological associationism.
With respect to animal diet, let it be considered that taking away the lives of animals, in order to convert them into food, does great violence to the priciples 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 animals. It is most evident in respect to the larger animals and htose with whom we have a familiar intercourse - such as oxen, sheep, and domestic fowls, etc. - 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, etc.; 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, etc., and if there be any glimmering 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. - Observations on Man (as quoted in The Extended Circle by Jon Wynne-Tyson)
Note: the following comment was found on an American Universiry website in respect of Percy Bysshe Shelley:
In July 1812, Shelley began writing one of his most famous poems Queen Mab . At this time he was concerned with education and was reeducating himself and preparing for his poem by studying a collection of Medical Extracts, Sir Humphrey Davy's Elements of Chemical Philosophy, Mary Wollstonecraft's Rights of Women, and an early psychological thesis, Observations of Man, by David Hartley. Queen Mab, however, was politics disguised as poetry. . . . Secondary themes were temperance, vegetarianism, and republicanism. What Shelley was preaching came to be understood as a "vision of the good life built on atheism, free love, republicanism, and vegetarianism."
It is also perhaps siginificant that Shelley himself became vegetarian at the beginning of March 1812, presumbaly whilst reading Hartley's book in preparation for Queen Mab.
It was also noted that Mary Wolstencraft Shelley used ideas from 'Observations on Man' in writing 'Frankenstein'.