Scientific critics say that Johnson exaggerates the deficiencies of Darwinian theory. Theories applying to the distant past can seldom be proved with certainty. But broad theories in science are judged in part by their ability to explain a wide range of data of differing kinds. Scientists compare alternative theories and evaluate their fruitfulness in suggesting further research, but Johnson offers no empirically testable alternative theory.[ix] Theological critics point out that he assumes that theism requires belief that God intervenes in gaps in the scientific account. They suggest that he has not adequately distinguished scientific theories themselves from the philosophical positions of their atheistic interpreters. So he has ended by agreeing with the exponents of evolutionary materialism that one cannot believe in both God and neo-Darwinism.
The biochemist Michael Behe argues that the "irreducible complexity" of biochemical systems shows that they cannot be the product of gradual evolution. He traces, for example, the long chains of molecular reactions that occur in the eye and in the immune system. He claims that such complex interlocking systems could not have had simpler functional precursors because they would not work if even a single step or component were missing. He offers the analogy of a mousetrap with five essential parts. If one part, such as the spring, were missing the trap would not work at all. It is an all-or-nothing system which must have been designed all at once, not by stages. "Since natural selection can only choose systems that are already working, then if a biological system cannot be produced gradually it would have to arise as an integrated unit, in one fell swoop, for natural selection to have anything to act on."[x].
Behe suggest that the information for designed systems might have been introduced in the earliest single cells and remained dormant for billions of years, or it might have been added later to produce a complex biochemical system.