Design necessitates a designer. This is a fundamental axiom. Thus, design detection methodology is a prerequisite for many fields of human endeavor, including archaeology, anthropology, forensics, criminal jurisprudence, copyright law, patent law, reverse engineering, crypto analysis, random number generation, and SETI (the Search for Extra-terrestrial Intelligence). In general, we find "specified complexity" to be a reliable indicator of the presence of intelligent design. Chance can explain complexity but not specification; a random sequence of letters is complex but not specified (it is meaningless). A Shakespearean sonnet is both complex and specified (it is meaningful). You can't have a Shakespearean sonnet without Shakespeare. (William A. Dembski, "The Design Inference: Eliminating Chance through Small Probabilities," 1998.)
When we apply the general principles of detecting specified complexity to living creatures, we find it reasonable to infer the presence of intelligent design. Common sense demands a Designer. Let's take the e-coli bacterial flagellum for example. The e-coli bacterial flagellum is what propels e-coli bacteria through their microscopic world. It consists of about 40 different protein parts (which come into focus when magnified 50,000 times using electron micrographs), including a stator, rotor, drive shaft, U-joint, and propeller. It is not simply convenient that we've given these parts these specific names - that's truly their function. The bacterial flagellum is a microscopic outboard motor! These microscopic outboard motors are absolutely amazing - a marvel of engineering. They can run at an incredible 100,000 rpm. Nevertheless, they can stop on a microscopic dime. In fact, it takes only a quarter turn for them to stop, shift gears and start spinning 100,000 rpm in the other direction! The flagellar motor is water-cooled and is hardwired into a signal transduction (sensory mechanism) so that it gets feedback from its environment! ("Unlocking the Mystery of Life," documentary by Illustra Media, 2002.)
The point is, if you were to find a stator, rotor, drive-shaft, U-joint, or propeller in any vehicle, any machine, any toy or model, you would recognize it as the product of an intelligent source. No one would expect any outboard motor, much less one this incredible, would ever be the product of a chance assemblage of parts. That is absurd. Outboard motors are the product of intelligent design. (Michael Behe, "Darwin's Black Box," 1996.)
The term "Irreducible Complexity" was first coined by Michael Behe in describing these molecular machines. Each mechanical part is absolutely necessary for the whole to function. Thus there is no naturalistic, gradual, evolutionary explanation for the existence of a bacterial flagellum. Not only does common sense demand a Designer, there is no plausible naturalistic explanation to explain away the necessity of a Designer.
The bacterial flagellum is only one among many thousands of intricate well-designed molecular machines. Molecular biologist Michael Denton wrote, "Although the tiniest bacterial cells are incredibly small, weighing less than 10-12 grams, each is in effect a veritable micro-miniaturized factory containing thousands of exquisitely designed pieces of intricate molecular machinery, made up altogether of one hundred thousand atoms, far more complicated than any machinery built by man and absolutely without parallel in the non-living world."
(Michael Denton, "Evolution: A Theory in Crisis," 1986, p. 250.)
The eye is a famous example of a supposedly irreducibly complex structure, due to its many elaborate and interlocking parts, seemingly all dependent upon one another. It is frequently cited by intelligent design and creationism advocates as an example of irreducible complexity. Behe used the "development of the eye problem" as evidence for intelligent design in Darwin's Black Box. Although Behe acknowledged that the evolution of the larger anatomical features of the eye have been well-explained, he claimed that the complexity of the minute biochemical reactions required at a molecular level for light sensitivity still defies explanation. Creationist Jonathan Sarfati has described the eye as evolutionary biologists' "greatest challenge as an example of superb 'irreducible complexity' in God's creation", specifically pointing to the supposed "vast complexity" required for transparency.