Why Evolution Works (and Creationism Fails)

Chapter 8: Why Intelligent-Design Creationism Fails

64 Chapter 8 Why Intelligent-Design Creationism Fails This isn’t really, and never has been, a debate about science. It’s about religion and philosophy. —Phillip Johnson, University of California Law School Intelligent-design creationism is the successor to scientific creationism such as that practiced by the flood geologists. ID creationists argue, in a variation on the argument from design, that they can rigorously infer design, whereas William Paley more or less assumed design.The heart of their argument is the claim that organisms are too complex to have evolved by chance. ID creationism has two main planks: a logical procedure called the explanatory filter, and analogies between living organisms and machines. The explanatory filter is said to never wrongly infer design where there is none. The analogy with machines is used to argue that cells have so many complex, interacting parts that they could not have been assembled,except by design.The two arguments are closely related and rely on intuitive notions about probability. Explanatory Filter The explanatory filter is a logical procedure that claims to use probability to infer whether an artifact is natural or designed. It is the brainchild of the mathematician William Dembski. Dembski claims that the filter can rigorously infer design and further, though it might sometimes miss an instance of design (a false negative), that it will never demonstrate design where there is none (a false positive). False negative and false positive are statistical terms:We get a false positive when we perform a test and incorrectly infer a positive result,as when we seem to detect tuberculosis in a person who in fact does not have tuberculosis. Similarly, we will get a false negative if we fail to detect tuberculosis in a person who has the disease. Dembski’s filter is shown in figure 2. To illustrate, let us apply it to some artifact, such as a snowflake or a mouse. Dembski calls the snowflake or the CH008.qxd 3/26/09 5:28 AM Page 64 Why Intelligent-Design Creationism Fails 65 mouse an event. If an event happens with regularity and very high probability, we attribute the event to a law and conclude that it is not designed. If the event happens with intermediate probability, then we attribute its occurrence to chance. If, however, the event happens with very low probability, less than 10⫺150 , and if it corresponds to a meaningful message (see below), then we attribute the event to design. The explanatory filter is, unfortunately, unusable. How high is high probability ? Dembski doesn’t say. How high is intermediate? Again, he doesn’t say. How, then, do we navigate the filter?We cannot get past the first branch point if we do not have a quantitative number for high probability. Additionally, Dembski provides no convincing proof that a probability of 10⫺150 necessarily infers design. It could, rather, infer “don’t know” or even “haven’t the foggiest.” Indeed, deciding that an event is designed may automatically prevent us from seeking and therefore finding the real cause. “Don’t know” is by far the better conclusion, because it is a spur to action: Find out. Finally, the filter is by no means immune to false positives. Here is a realworld example in which the police, in effect, applied the explanatory filter with tragic results. A British woman lost three babies to sudden infant death syndrome (SIDS, or crib death) within four years. The Crown Prosecution Service reasoned as follows: One death is tragic; two deaths are suspicious; three deaths are murder.The woman was prosecuted. What the prosecutors did not know or ignored was that SIDS may be a genetic disease that runs in families. Indeed, the woman’s grandmother testified that three of her own children had died of unexplained causes before the ages of six weeks (in the 1940s, before SIDS had been recognized). A geneticist testified that SIDS could indeed run in families and suggested two Yes Yes No No High probability Intermediate probability Low probability Law Chance Design Yes 2. Dembski’s explanatory filter. The probabilities are not given, so the filter cannot be used in practice. CH008.qxd 3/26/09 5:28 AM Page 65 possible mechanisms. The jury used this background information—what Dembski calls side information—and acquitted the woman. In short, the prosecutors applied the explanatory filter and very probably got a false positive, whereas the jury...