intelligent design, theory that some complex biological structures and other aspects of nature show evidence of having been designed by an intelligence. Such biological structures are said to have intricate components that are so highly interdependent and so essential to a particular function or process that the structures could not have developed through Darwinian evolution, and therefore must have been created or somehow guided in their development. Although intelligent design is distinguished from creationism by not relying on the biblical account of creation, it is compatible with a belief in God and is often explicitly linked with such a belief. Also, unlike creationists, its proponents do not challenge the idea that the earth is billions of years old and that life on earth has evolved to some degree. The theory does, however, necessarily reject standard science's reliance on explaining the natural world only through undirected natural causes, believing that any theory that relies on such causes alone is incapable of explaining how all biological structures and processes arose. Thus, despite claims by members of the intelligent-design movement that it is a scientific research program, the work of its adherents has been criticized as unscientific and speculative for inferring a pre-existing intelligence to explain the development of biological structures instead of attempting to develop adequate falsifiable mechanistic explanations. In addition, the theory has been attacked on the grounds that many aspects of nature fail to show any evidence of intelligent design, such as "junk" DNA (see nucleic acid) and the vestigial webbed feet of the frigate bird (which never lands on water).

The idea that nature shows signs of having been designed by an intelligent being dates back at least to ancient Greece. The English theologian William Paley gave the theory its classic formulation in his Natural Theology; or, Evidences of the Existence and Attributes of the Deity (1802), in which he argued that the eye and other biological features are perfectly suited for their purposes and that in this suitable design the hand of God can be discerned. The modern intelligent-design movement, however, has its origins in the 1980s with such works as The Mystery of Life's Origins (1984) by Charles Thaxton et al. and Evolution: A Theory in Crisis (1986) by Michael Denton. Micheal Behe's Darwin's Black Box (1996) is perhaps the best-known statement of the movement's critique of Darwin and its argument for a role for God or some other intelligence in the design of biological entities. Advocates of intelligent design have campaigned to have it taught in U.S. public schools alongside the Darwinian theory of evolution. A requirement by the Dover, Pa., area school board that students be told that intelligent design represents an alternative explanation for the origin of life was challenged in federal court in 2005 and ruled unconstitutional.

Argument intended to demonstrate that living organisms were created in more or less their present forms by an “intelligent designer.” Intelligent design was formulated in the 1990s, primarily in the United States, as an explicit refutation of the Darwinian theory of biological evolution. Building on a version of the argument from design for the existence of God, proponents of intelligent design observed that the functional parts and systems of living organisms are “irreducibly complex” in the sense that none of their component parts can be removed without causing the whole system to cease functioning. From this premise they inferred that no such system could have come about through the gradual alteration of functioning precursor systems by means of random mutation and natural selection, as the standard evolutionary account maintains; therefore, living organisms must have been created all at once by an intelligent designer. Proponents of intelligent design generally avoided identifying the designer with the God of Christianity or other monotheistic religions, in part because they wished the doctrine to be taught as a legitimate scientific alternative to evolution in public schools in the United States, where the government is constitutionally prohibited from promoting religion. Critics of intelligent design argued that it rests on a fundamental misunderstanding of natural selection, that it ignores the existence of precursor systems in the evolutionary history of numerous organisms, and that it is ultimately untestable and therefore not scientific. Seealso creationism.

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