 In the 18th and 19th centuries, Hartley’s Observations was valued very highly by people within the realm of religious dissent, scientific progress, and social reform. Writing in 1774, Joseph Priestley, a leading Unitarian and the foremost scientist in Britain, wrote that the Observations “contains a new and most extensive science” and promised that “the study of it … will be like entering upon a new world.” He added: “I think myself more indebted to this one treatise, than to all the books I ever read beside; the scriptures excepted” (1774, xix).