Michael Behe defined irreducible complexity in natural selection in terms of well-matched parts in his 1996 book Darwin's Black Box:
“... a single system which is composed of several well-matched, interacting parts that contribute to the basic function, and where the removal of any one of the parts causes the system to effectively cease functioning.”
A second definition given by Behe in 2000 (his ‘evolutionary definition’) states:
“An irreducibly complex evolutionary pathway is one that contains one or more unselected steps (that is, one or more necessary-but-unselected mutations). The degree of irreducible complexity is the number of unselected steps in the pathway.”
Intelligent-design advocate William A. Dembski assumed an ‘original function’ in his 2002 definition:
“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.”
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