Natural Theology, published by the English clergyman William Paley in 1802, sets out what remains the best known statement of the argument from design for God's existence, built around a single extended analogy: a person crossing a heath who stumbles upon a watch would reasonably infer a watchmaker from its intricate, purposeful arrangement of parts, and Paley argues that the far greater intricacy and apparent purposefulness observed throughout the natural world, in the structure of the eye, the workings of living bodies and the interlocking arrangement of the natural order generally, warrants the same inference on a vastly larger scale, pointing to a divine designer behind nature as a whole.
Paley develops the analogy across an extended survey of anatomical and natural detail, treating the argument less as a single syllogism than as a cumulative case built from example after example of apparent design in living structures, an approach that gave the book significant persuasive weight for its original readership as a work of natural science as much as of theology.
Paley did not originate the design analogy himself; versions of the watch and watchmaker comparison circulated among earlier natural theologians, but his particular formulation, developed at length and with extensive supporting natural historical detail, became the version later readers and critics engaged with most directly, effectively fixing the analogy's classic form for subsequent philosophical discussion.