The watchmaker analogy is a version of the design argument for God's existence given its best known English statement by William Paley in his 1802 book Natural Theology. Paley asks the reader to imagine crossing a heath and finding a stone, which could plausibly have lain there forever without implying any maker, against finding a watch, whose many parts are arranged and fitted together for the specific purpose of telling time in a way that, Paley argues, could not plausibly have arisen by chance, forcing the conclusion that the watch had a maker, an intelligent designer who understood its purpose and constructed it accordingly.
Paley then applies the same reasoning to living organisms, arguing that biological structures such as the eye display a comparable, indeed far greater, complexity and fittedness to purpose than any watch, and that this complexity equally implies a designer, in this case God, whose attributes as creator Paley spends the rest of the book cataloguing from the evidence of the natural world. The argument was widely taught and highly influential in English speaking Christian thought through the early nineteenth century, including on the young Charles Darwin, who read Paley admiringly as a student before his own later theory offered a very different explanation for the same observed complexity.