Paley's Principles of Moral and Political Philosophy was one of the most influential philosophical texts in late Enlightenment Britain. It was cited in several parliamentary debates over the corn laws in Britain and in debates in the US Congress. The book remained a set textbook at Cambridge well into the Victorian era. Even Charles Darwin was required to read it when he studied at Christ's College. But it was Paley's Natural Theology that most impressed Darwin, even though it was not a set book for undergraduates.
Paley is also remembered for his contributions to the philosophy of religion, utilitarian ethics and Christian apologetics. In 1802, he published Natural Theology; or, Evidences of the Existence and Attributes of the Deity, his last book. As he states in the preface, he saw the book as a preamble to his other philosophical and theological books; in fact, he suggests that Natural Theology should be read first, so as to build a systematic understanding of his arguments. The main thrust of his argument was that God's design of the whole creation could be seen in the general happiness, or well-being, that was evident in the physical and social order of things. Such a book fell within the broad tradition of natural theology works written during the Enlightenment; and this explains why Paley based much of his thought on John Ray (1691), William Derham (1711) and Bernard Nieuwentyt (1750).
Paley's argument is built mainly around anatomy and natural history. "For my part", he says, "I take my stand in human anatomy"; elsewhere he insists upon "the necessity, in each particular case, of an intelligent designing mind for the contriving and determining of the forms which organized bodies bear". In making his argument, Paley employed a wide variety of metaphors and analogies. Perhaps the most famous is his analogy between a watch and the world. Historians, philosophers and theologians often call this the Watchmaker analogy. Building on this mechanical analogy, Paley presents examples from planetary astronomy and argues that the regular movements of the solar system resemble the workings of a giant clock. To bolster his views he cites the work of his old friend John Law and the Dublin Astronomer Royal John Brinkley.
The germ of the idea is to be found in ancient writers who used sundials and Ptolemaic epicycles to illustrate the divine order of the world. These types of examples can be seen in the work of the ancient philosopher Cicero, especially in his De natura deorum, ii. 87 and 97. The watch analogy was widely used in the Enlightenment, by deists and Christians alike. Thus, Paley's use of the watch (and other mechanical objects like it) continued a long and fruitful tradition of analogical reasoning that was well received by those who read Natural Theology when it was published in 1802.