A friend asked about this common intelligent design concept. Specified complexity, also called complex specified information (CSI):
Life shows evidence of complex, aperiodic, and specified information in its key functional macromolecules, and the only other example we know of such function-specifying complex information are artifacts designed by intelligent agents. A chance origin of life would exceed the universal probability bound (UPB) set by the scope of the universe; hence design is a factor in the origin and development of life. Contrary to a commonly encountered (and usually dismissive) opinion, this concept is neither original to Dr Dembski nor to the design theory movement. Its first recognized use was by noted Origin of Life researcher, Leslie Orgel, in 1973:
Living organisms are distinguished by their specified complexity. Crystals fail to qualify as living because they lack complexity; mixtures of random polymers fail to qualify because they lack specificity. [ L.E. Orgel, 1973. The Origins of Life. New York: John Wiley, p. 189.
The concept of complex specified information helps us understand the difference between (a) the highly informational, highly contingent aperiodic functional macromolecules of life and (b) regular crystals formed through forces of mechanical necessity, or (c) random polymer strings. In so doing, they identified a very familiar concept — at least to those of us with hardware or software engineering design and development or troubleshooting experience and knowledge. Furthermore, on massive experience, such CSI reliably points to intelligent design when we see it in cases where we independently know the origin story. More.
Here are some suggested resources:
Bill Dembski: Specification: The Pattern That Signifies Intelligence
Kirk K Durston et al. Measuring the functional sequence complexity of proteins
Winston Ewert at Evolutionary Informatics
Robert M. Hazen et al. Functional information and the emergence of biocomplexity (public access) A friend notes, “Functional information, as outlined by Hazen et al., can be a measure of specified complexity, where the specificity supplies the functional constraint.”
Robert M. Hazen et al. Functional Information and the Emergence of Biocomplexity pdf (book)
Note: Another term used around here is irreducible complexity:
Irreducible Complexity, IC — A system performing a given basic function is irreducibly complex if it includes a set of well-matched, mutually interacting, nonarbitrarily individuated parts such that each part in the set is indispensable to maintaining the system’s basic, and therefore original, function. The set of these indispensable parts is known as the irreducible core of the system. (Dembski, No Free Lunch, p. 285
See also: Could a signature of specified complexity help us find alien life?
or
A Tutorial on Specified Complexity