In a series of five major metaphysical works, all written between 1774 and 1778, Priestley laid out his materialist view of the world and tried "to defend Christianity by making its metaphysical framework more intelligible," even though such a position "entailed denial of free will and the soul." The first major work to address these issues was "The Examination of Dr. Reid's Inquiry ... Dr. Beattie's Essay ... and Dr. Oswald's Appeal" (1774). He challenged Scottish common-sense philosophy, which claimed that "common sense" trumped reason in matters of religion. Relying on Locke and Hartley's associationism, he argued strenuously against Reid's theory of mind and maintained that ideas did not have to resemble their referents in the world; ideas for Priestley were not pictures in the mind but rather causal associations. From these arguments, Priestley concluded that "ideas and objects must be of the same substance," a radically materialist view at the time. The book was popular and readers of all persuasions read it. Charles Lamb wrote to Samuel Taylor Coleridge, recommending "that clear, strong, humorous, most entertaining piece of reasoning" and Priestley heard rumors that even Hume had read the work and "declared that the manner of the work was proper, as the argument was unanswerable."