One of, if not the, most famous arguments for Intelligent Design Theory, or Argument From Design, originally put forth by William Paley in Natural Theology - or Evidences of the Existence and Attributes of the Deity Collected from the Appearances of Nature, published in 1802. Although Paley himself never referred to a Watchmaker Theory by name, his comparison of life to a complicated and intelligently designed watch has become well-known and used by Creationists from every religion.

In the beginning of Natural Theology, Paley states a difference between simple, physically homogeneous objects such as stones, and purposeful, complex, manufactured objects like watches, and believes that for a watch to come into being "... the watch must have had a maker: that there must have existed, at some time, and at some place or other, an artificer or artificers, who formed it for the purpose which we find it actually to answer; who comprehended its construction, and designed its use... every indication of contrivance, every manifestation of design, which existed in the watch, exists in the work of nature; with the difference, on the side of nature, of being greater or more, and that in a degree which exceeds all comparison."*

Paley's first example of a supposedly perfect biological mechanism is the human eye, and this has remained a staple of creationist debate for two hundred years now. He argues that the eye was obviously designed for the purpose of vision, as telescope is designed for enhancing vision. Creationists since Darwin's time have taken this argument one step further and stated (erroneously) that a partially evolved eye would be of no benefit to an animal, and that the various mechanisms in an eye could not have separately evolved as they depend on one another.

What the Watchmaker Theory, and especially the example of the eye, ignore is that evolution is not merely a process of blind and random change. On the contrary, natural selection dictates that any variation which confers an advantage to a life-form will be passed on and magnified for future generations. Darwin explains that many different levels of complexity can be observed in the eyes of living crustaceans, and that there may be an advantage gained from simple light sensitivy. He suggests that any simple nerve can, with simple modifications, be made sensitive to light, and finally that "... if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real."**

* - William Paley in Natural Theology, as quoted in "The Blind Watchmaker" by Richard Dawkins.
** - Charles Darwin, "The Origin of Species".