The objection to Behe’s argument that I want to focus on here concerns the type of reasoning he employs against evolutionary theory and in favor of the hypothesis of intelligent design. Behe repeatedly vacillates between using a deductive and a probabilistic modus tollens against evolutionary theory. The vacillation sometimes occurs on the same page. Consider the following passage:
... I have shown why many biochemical systems cannot be built up by natural selection workingon mutations: no direct, gradual route exists to these irreducibly complex systems ... There is nomagic point of irreducible complexity at which Darwinism is logically impossible. But the hurdlesfor gradualism become higher and higher as structures are more complex, more interdependent
(p. 203).
Behe’s first sentence says that irreducible complexity cannot arise by Darwinian processes; however, the next two assert, more modestly, that irreducibly complex features are improbable on the Darwinian model and that they become more improbable the more complex they are. I hope it is clear from what I’ve said earlier why this shift is important. If evolutionary theory really did have the deductive consequence that organisms cannot have features that are irreducibly complex, then that theory would have to be false, if such features exist. But what if the theory merely entailed that irreducibly complex
features are very improbable? Would the existence of such features show that the theory is
improbable? Would it follow that the theory is disconfirmed by those observations? Would it followthat these features provide evidence in favor of intelligent design? The answers to all these questions arethe same – no. There is no probabilistic analog of modus tollens.
In addition to rejecting evolutionary explanations, Behe advances the positive thesis that the
biochemical systems he describes in loving detail “were designed by an intelligent agent” (p. 204).
However, for these details to favor intelligent design over mindless evolution, we must know how probable those details are under each hypothesis. This is the point of the Law of Likelihood. Behe asserts that these details are very improbable according to evolutionary theory, but how probable are they according to the hypothesis of intelligent design? It is here that we encounter a great silence. Behe and other ID theorists spend a great deal of time criticizing evolutionary theory, but they don’t take even the first steps towards formulating an alternative theory of their own that confers probabilities on what we observe. If an intelligent designer built the vertebrate eye ,or the bacterial flagellum, or the biochemical cascade that causes blood to clot, what is the probability that these devices would have thefeatures we observe? The answer is simple – we do not know. We lack knowledge of what this
putative designer’s intentions would be if he set his mind to constructing structures that perform these functions.
The sad fact about ID theory is that there is no such theory. Behe argues that evolutionary theory entails that adaptive complexity is very improbable, Johnson rails against the dogmatism of scientists who rule out a priori the possibility of supernatural explanation, and Dembski tries to construct an epistemology in which it is possible to gain evidence for the hypothesis of design without ever having to know what, if anything, that hypothesis predicts. A lot goes wrong in each of these efforts but notice what is not even on the list.
Intelligent design theorists may feel that they have already stated their theory. If the existence of the vertebrate eye is what one wishes to explain, their hypothesis is that an intelligent designer constructed the vertebrate eye. If it is the characteristics of the vertebrate eye (the fact that it has features F1, F2,...FN rather than its mere existence, that one wants to explain, their hypothesis is that an intelligent designer constructed the vertebrate eye with the intention that it have features F1, F2,...FN and that this designer had the ability to bring his plan to fruition.
Notice that both of these formulations of the hypothesis of intelligent design simply build into that hypothesis the observations whose explanation we seek. The problem with this strategy is that the same game can be played by the other side. If theevolutionary hypothesis is formulated by saying “evolution by natural selection produced the vertebrate
eye” or by saying that “evolution by natural selection endowed the eye with feature F1, F2,...FN then it too entails the observations.
To avoid trivializing the problem in this way, we should formulate the observations so that they
are not built into the hypotheses we want to test. This can be achieved by organizing the problem as follows:
(O) The vertebrate eye has features F1, F2,...FN
(ID)
The vertebrate eye was created by an intelligent designer.
(ENS) The vertebrate eye was the result of evolution by natural selection.
Behe claims that (O) has a low probability according to the (ENS) hypothesis.
My complaintis that we do not know what the probability of (O) is according to (ID). If an intelligent designer made the eye, perhaps he would have been loathe to give it the features we observe. Or perhaps he would have aimed at producing those very characteristics.
The single sentence stated in (ID) [EA: The designer did it] does not a theory make. This problem is not solved by simply inventing assumptions about the putative designer’s
goals and abilities; what is needed is information about the putative designer(s) that is independently attested. Without that information, the theory makes no predictions about the eye or about the other examples of “irreducible complexity” that Behe discusses. And without those predictions, the intelligent design movement can provide no evidence against the evolutionary hypothesis.
After concluding that evolutionary theory cannot explain adaptations that are irreducibly
complex, Behe briefly broaches the subject of whether some “as-yet-undiscovered natural process” might be the explanation. Here is his analysis:
No one would be foolish enough to categorically deny the possibility ... [however] if there is
such a process, no one has a clue how it would work. Further, it would go against all human
experience, like postulating that a natural process might explain computers ... In the face of the massive evidence we do have for biochemical design, ignoring that evidence in the name of aphantom process would be to play the role of the detectives who ignore an elephant (pp. 203-204).
Notice that Behe claims that there is “massive evidence for biochemical design,” but what is that evidence? It seems to consist of two facts, or alleged facts – that evolutionary theory says that irreducibly complex adaptations have low probabilities and that no one has yet formulated any other theory restricted to mindless natural processes that could be the explanation. However, if the comparative principle about evidence stated earlier is correct, this “evidence” is no evidence at all.
After evolutionary theory and “as-yet-undiscovered natural process[es]” are swept from the
field, Behe immediately concludes that the biological mechanisms whose details he has described... were designed by an intelligent agent.
We can be as confident of our conclusion for these cases as we are of the conclusions that a mousetrap was designed, or that Mt. Rushmore or an Elvis poster were designed ...Our ability to be confident of the design of the cilium or intracellular transport rests on the same principles as our ability to be confident of the design of anything: the ordering of separate components to achieve an identifiable function that depends sharply on those the components (p. 204).
Behe is right that the nonbiological examples he cites favor hypotheses of intelligent design over hypotheses that postulate strictly mindless natural processes, but he is wrong about the reason and wrong to think that biochemical adaptations can be assimilated to the same pattern. In the case of mousetraps, Mount Rushmore, and Elvis posters, we are confident about intelligent design because we have strong evidence for human intelligent design. We know that all of these objects are just the sorts of things that human beings are apt to make. The probability of their having the features we observe, on the hypothesis that they were made by intelligent human designers, is fairly large, whereas the probability
of their having those features, if they originated by chance, is low. The likelihood inference is
unproblematic. But the probability that the bacterial flagellum would have the features we observe, orthat the mechanism for blood clotting would have its observed features, if human beings somehow made those devices, is very very low.
ID theorists therefore are led to consider possible nonhuman designers – indeed, possible designers who are supernatural. Some of these possibilia would, if they
existed, have goals and abilities that would make it highly probable that these devices have the featureswe observe; others would not. Averaging over all these possibilities, what is the probability that the device will have the features we observe if it was made by some possible intelligent designer or other?
We do not know, even approximately.
Behe would like to be able to identify an observable feature of natural objects that could not
exist if those objects were produced by strictly mindless processes and that therefore must be due to intelligent design (natural or supernatural). There is no such property. It is not impossible for irreducibly complex functional features to arise by the evolutionary process of natural selection, which is not a random process.
Indeed, it isn’t even impossible for them to arise by a purely random chance process. This is the simple point made vivid by thinking about monkeys and typewriters and of particles
whirling in the void.
The next step is to think about the properties that an object probably will have if it is made by an intelligent designer and probably won’t have if it isn’t. The problem here is that there are
many kinds of possible intelligent designers, and many kinds of possible mindless processes. Is there a property that a natural object probably will have, no matter what sort of possible intelligent designer made it? I am confident that the answer to this question is no. Is there a property that it probably won’t have, no matter what sort of possible mindless process made it?
As for this second question, here I am in agreement with Behe – we really don’t know. But ignorance does not constitute a reason to reject the possibility that what we observe is due to mindless natural processes that we have not yet considered and conclude that what we observe must be due to intelligent design