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Bacterial Flagella and Dembski’s Case for Intelligent Design: Closing Arguments

Earlier in this critique we outlined Dembski’s case for the proposition - bacterial flagella are intelligently designed - as follows:

  1. The bacterial flagellum displays specified complexity.

  2. Specified complexity in biotic systems cannot be generated by the Darwinian mechanism, which relies on chance.

  3. Therefore the bacterial flagellum must have been intelligently designed - that is, it could have been actualized only with the assistance of form-conferring interventions by an unembodied intelligent agent.

We then focused our attention on the first of these statements and asked, Does the bacterial flagellum exhibit “specified complexity” as it is defined by Dembski? In order to do that it would have to be a) sufficiently complex, and b) specified.

Is the bacterial flagellum sufficiently complex? Using Dembski’s own criterion, only if the probability of its being actualized by the joint action of all natural causes is less than the universal probability bound. Dembski attempted to demonstrate this to be true by treating the bacterial flagellum as if it were a discrete combinatorial object actualized by the pure chance gathering of 50 of the right kinds of proteins (and in the correct proportions) at some spot in the vicinity of the cell wall and plasma membrane of E. coli and then, again by chance, happening to configure themselves into a functioning rotary propulsion system for this bacterial cell. The only natural formational process that Dembski considered in his probability computation was self-assembly by pure happenstance.

We reject that argument as being a totally unrealistic caricature of how the flagellum is actualized and an approach that totally ignores the role of the bacterial genome in coding for all of the structures and functions that contribute to the nature of E. coli. E. coli bacteria possess flagella, not because flagella self-assemble and self-attach to the cell membrane, but because the genome of E. coli came to include in its genetic library the coded instructions for growing the flagellar propulsion system. Dembski’s case for the complexity (as he defines it) of the bacterial flagellum fails.

Is the bacterial flagellum specified? Using Dembski’s own criterion, only if it exhibits a pattern that is detachable - wholly independent of the event that produced it. Appearing to set aside his laboriously crafted formalism regarding the specification and detachability requirements, Dembski simply asserts that in the case of biological systems specification always refers to function, and declares that biological functions are inherently detachable from the particular biological systems that instantiate them.

We reject that argument for a number of reasons. First, the general principle that biological function counts for specification was never established by Dembski. Second, his application of this principle appears to be entirely ad hoc. Dembski provides no systematic means for concluding that the function of the flagellum should count as a detachable specification while other equally remarkable biological functions of E. coli go unmentioned. If none of the other biological functions of the bacterium count as sufficiently remarkable to serve as a detachable specification, then neither should the function of the flagellum. Dembski’s case for the specification (as he defines it) of the flagellum fails.

Finally, there is the broad question concerning Dembski’s rhetorical use of key terms like complexity and specification. On the question of specification, for instance, Dembski asserts that, “no biologist I know questions whether the functional systems that arise in biology are specified.”NFL, p. 289.The question is, however, Is Dembski using the term “specified” in the same way as the biologists he has in mind? The answer, I believe, is, No. Dembski treats the presence of biological function as if it constituted a detachable pattern independent of the organism under scrutiny. For Dembski, biological function is one of the qualities of a complex organism that only intelligent intervention could produce. For biology, on the other hand, biological function plays nearly the opposite role. Biological function is the very capacity of an organism that gives it the ability to respond to its environment in the manner described by a rich menu of dynamically changing fitness functions, a responsive phenomenon that lies at the heart of evolutionary dynamics.

A similar concern about Dembski’s use of key terms arises in regard to the meaning of “complexity.” After quoting a number of prominent biologists regarding the scientific challenge of accounting for the information now resident in complex biological systems, Dembski asks, “But what sort of information are they talking about?”NFL, p. 148.He soon answers his question in a way that appears to place their concern squarely in his own design-theoretic court. “I submit that what they have in mind is specified complexity, or what equivalently we have been calling ... complex specified information. Certainly the complexity of biological information is not at issue.”NFL, p. 148.But of course this bold assertion could be true only if Dembski is using the term “complexity” in the same way as the persons he quoted, which seems not to be the case. As we saw in our general considerations on complexity, the “complexity” that Dembski computes is a property, not of some biotic system itself, but of the means by which it becomes actualized. That is why, for instance, he treated the bacterial flagellum as if it were a chance-assembled discrete combinatorial object and judged its complexity on the basis of the probability of its coming to be actualized by nothing more than pure accident. Most biologists, on the other hand, use the term “complexity” as the name of something quite different - a structural or functional quality of the biotic system itself.

But “specification” and “complexity” are not the only terms that Dembski, like other leaders of the ID movement, employs rhetorically with unorthodox meanings. Recall, for instance, that to be “intelligently designed” is, in effect, to be assembled with the aid of form-conferring (or information-infusing) action performed by an unnamed and unembodied choice-making agent. Recall also that when this unembodied intelligent agent brings about a naturally impossible outcome, it is not a “miracle.” And recall that “chance hypothesis” most often means all hypotheses, postulates and theories concerning the natural causation of events. The case for ID relies on a web of words that have been assigned extraordinarily unusual meanings.

Given this character trait of ID literature, including such works as Dembski’s No Free Lunch, would it not be appropriate to suggest that Dembski pause to reflect on his own admonition regarding the need to use words appropriately, consistently, and with precision to avoid the charge of equivocation? Says Dembski, “The fallacy of equivocation is the fallacy of speaking out of both sides of your mouth. It is the deliberate confusing of two senses of a term, using the sense that’s convenient to one’s agenda.”William A. Dembski, Intelligent Design: The Bridge Between Science & Theology (Downers grove, IL: InterVarsity Press, 1999), p. 115.

On this point Dembski and I agree. Ironically, however, I find Dembski’s rhetoric to be riddled with the very equivocation that he condemns.

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E. Coli at the No Free Lunchroom

Bacterial Flagella and Dembski’s Case for Intelligent Design: Closing Arguments

The Core of Dembski’s Case for ID
Why focus on the bacterial flagellum?
Getting Acquainted With the ID Vocabulary
Doing what comes naturally
Darwinism = evolution + maximal naturalism
The Darwinian mechanism
What does it mean to be “intelligently designed”?
The signs of design
E. coli and its Rotary Propulsion System: Dembski’s Flagship Case for Design
Is the flagellum complex? General considerations
Is the flagellum complex? Computing the crucial probability.
Is the flagellum specified?


Howard Van Till
Dr. Howard Van Till

See also:

Dembski: Intelligent Design Coming Clean...
Purpose and Design
Charles Darwin
Bacterial Flagellum
DNA Double-Helix
Books on Biology, Genetics and Theology