The signs of design

How would we come to know that something was intelligently designed? It’s very straightforward, says Dembski.

There does in fact exist a rigorous criterion for discriminating intelligently caused from unintelligently caused objects. ... I call it the complexity-specification criterion. When intelligent agents act, they leave behind a characteristic trademark or signature - what I define as specified complexity. The complexity-specification criterion detects design by identifying this trademark of designed objects.

Whenever we infer design we must establish three things: contingency, complexity, and specification.

An object/event is said to be contingent if, while it is fully consistent with natural laws, it is not wholly determined by them and represents only one outcome among several possible outcomes of natural processes. Complexity is related inversely to probability. Highly complex objects have a low probability of being actualized naturally. Dembski looks for objects whose probability of actualization by natural means is less than what he calls the “universal probability bound,” which has the value 10 ^{- 150}. For some event/object to be specified it must exhibit a distinctive pattern that is detachable from the particular event/object itself. A detachable pattern might, for instance, correspond to some independently derivable sequence of numbers or letters that has no necessary connection to the object/event being subjected to the complexity-specification criterion. For example, if SETI researchers received a radio signal representing the first 100 prime numbers they would be justified in concluding that the signal exhibited a detachable pattern that had no necessary relationship to the electromagnetic waves that carried it.

In Dembski’s language, if some event/object is contingent (not the outcome of any deterministic natural law), and sufficiently complex (its probability of natural actualization is less than 10 ^{- 150}), and specified, then it exhibits specified complexity. The central argument of No Free Lunch is that objects/events that exhibit specified complexity cannot be actualized by natural processes alone and must, therefore, be the outcome of intelligent design, in the sense consistent with the way in which all of the key terms have been defined above.

Establishing the contingency of some event/object is ordinarily a rather simple matter. Establishing complexity and specification, however, is difficult (perhaps impossible), as our case study of the bacterial flagellum will illustrate.

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