Intelligent design (ID) is the assertion that empirical evidence supports the conclusion that the initial life on earth, and perhaps some of its present details, was deliberately designed by one or more intelligent agents; additionally, or alternately, it may include the idea that different empirical evidence supports a similar conclusion regarding the universe itself.
The phrase "intelligent design," used in this sense, was first widely publicized by legal scholar Phillip E. Johnson in his 1991 book Darwin on Trial, although earlier references can be found in creationist literature. Johnson's assertion, and a key tenet of the ID movement, is Theistic realism, and the rejection of philosophical naturalism.
Some ID proponents argue that the standard scientific model of evolution by natural selection is insufficient to explain the origin, complexity, and diversity of life, in light of instances of irreducible complexity and specified complexity. Some, in a view that may be compatible with evolution, argue that the universe is "fine tuned" for living things in a manner that must have been by design.
ID makes no explicit claims about the identity of the designer, but asserts that the motive was to create or make life possible and that the method is some form of applied intelligence.
The Intelligent Design movement is an organized campaign to promote ID arguments in the public sphere, primarily in the United States. The hub of the movement is the Center for Science and Culture, a subsidiary of the Discovery Institute, a politically conservative think tank. Though explicitly secular in its arguments, the ID movement is associated with some conservative Christians, hence ID is sometimes described as a revision of the argument from design made famous by William Paley in the early 19th century.
The most common response of ID's opponents in the scientific community is to reject the ID claims as being scientifically illegitimate. For example, the National Academy of Sciences and the National Center for Science Education have described ID as pseudoscience. Nonetheless, the ID movement has attracted a contingent who advocate teaching about ID in school science curricula, on grounds of full disclosure regarding perceived shortcomings of Darwinian evolution, and counteracting what they perceive to be an institutionalized atheistic presumption of much of the scientific establishment, and thereby promoting religious neutrality (see Creation and evolution in public education). Critics have in turn labeled ID "stealth creationism," a veiled attempt to introduce religious beliefs into scientific discourse. They argue that it is merely designed to circumvent already existing legal protections against establishment of religion in previous Supreme Court cases, and that ID advocates are religious zealots bent on polluting the discourse. They point to the many associations between ID and the religious right, and numerous examples of ID proponents making accusations of "bigotry" as proof that ID is not a scientific movement.
Thus, the subject of Intelligent Design is deeply embedded in political controversies, with charges of dogmatic bias and bad faith being made on all sides.
The term "irreducible complexity" was coined by biochemist Michael Behe in his 1996 book Darwin's Black Box. The irreducible complexity argument holds that evolutionary mechanisms cannot account for the emergence of some complex biochemical cellular systems. ID advocates argue that the systems must therefore have been deliberately engineered by some form of intelligence.
"Irreducible Complexity" is defined by Behe as "a single system which is composed of several well-matched interacting parts that contribute to the basic function, wherein the removal of any one of the parts causes the system to effectively cease functioning" (Behe, Molecular Machines: Experimental Support for the Design Inference). According to the theory of evolution, genetic variations occur without specific design or intent, and the environment selects variants that have the highest fitness, which are then passed on to the next generation of organisms; change occurs by the gradual operation of natural forces over time, perhaps slowly, perhaps more quickly (See punctuated equilibrium) and is able to create complex structures from simpler beginnings, or convert complex structures from one function to another (see spandrel). Most ID advocates accept that evolution through mutation and natural selection occurs, but assert that it cannot account for irreducible complexity, because none of the parts of an irreducible system would be functional or advantageous until the entire system is in place.
Behe uses the mousetrap as an illustrative example of this concept. A mousetrap consists of several interacting pieces—the base, the catch, the spring, the hammer—all of which must be in place for the mousetrap to work. The removal of any one piece destroys the function of the mousetrap. Likewise, biological systems require multiple parts working together in order to function. Natural selection could not create from scratch those systems for which it is impossible to find a viable evolutionary pathway of successive, slight modifications, because the selectable function is only present when all parts are assembled. Behe's original examples of irreducibly complex mechanisms included the bacterial flagellum of E. coli, the blood clotting cascade, cilia, and the adaptive immune system.