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Mark Perakh's Web Site

An account of how we detect design

By Mark Frank

Posted March 31, 2006

The intelligent design (ID) movement claims to have a formal method for detecting design by observing the results of the design process (this form of words get tedious so I will use the word "outcome" to indicate one or more events or objects that are the consequence of a designer at work). ID proponents have biology in mind, but they claim their approach works for detecting design in any sphere. There are many (including me) who disagree with the ID movement, but it is true that we can, and often do, detect design by observing the outcome. We can deduce with great certainty that Stonehenge is designed and not just a pile of rocks, and that an elaborate sandcastle is designed and not just the result of the tide. We can do this by inspecting the results -- not by observing the designer at work. So anyone who wants to criticise ID ought to explain how we make this deduction, what makes it reasonable, and how it differs from the ID approach.

The ID inference is very simple in essence. ID proposes that outcomes may be the result of either necessity (the sun must rise because of universal natural laws); chance (it was bad luck that Katrina struck New Orleans); or design (the planes struck the World Trade Centre because that was the terrorists' intention). Therefore, if we can rule out necessity or chance then the cause must be design. In fact most of the complexity in ID is deciding the rules for when chance can be ruled out. ID rules out chance on the basis that if the outcome is too improbable on the basis of chance then it cannot have been caused by chance. When the first plane struck the World Trade Center I think most observers put it down to an awful accident. When the second plane struck most spectators realised it was a deliberate plot. The chances of two planes accidentally hitting the World Trade Centre on the same day were so small as to be effectively impossible. The events of September 11th are a graphic illustration of the ID argument -- some things are just too improbable to be accidents.

Now consider a different scenario. Suppose the twin towers had been hit not by planes but by two meteors, large enough to seriously damage the towers, and for the purposes of this fantasy suppose the meteors had no physical connection that we could discern -- they were from different sources and were following unrelated trajectories. Such meteor strikes are less frequent than commercial airplanes flying into buildings -- remember Schipol. So the probability of two such meteors striking the same location in the same day is even smaller than two planes striking the same location. Yet we would hesitate to ascribe the two meteors to design. We would certainly be less inclined to ascribe them to design than the two planes. We would ask questions such as "how could anyone control where the meteors land?"Instead of ascribing design, we would almost certainly want to look more deeply into the causes of these meteors to see if there was some factor that could explain why these two freakish events happened at the same place on the same day.

Can we describe this reasoning more formally and does it justify our intuitive reaction? I think we can.

First is important to recognise that when we decide between the causes of outcomes we compare the plausibility of possible causes (this sounds trivial but I believe it is vital -- so hold on to it). Let's first look at the process of comparing causes when design is not under consideration. For example, I might be trying to decide whether a damp patch is due to condensation or a leaky roof. I would do this by considering two things: how likely is it that each possible cause (condensation or a leaky roof) would lead to that particular damp patch in that location and how likely is it that there is condensation or a leaky roof in the first place. For example, I might start off thinking: "that is where I would expect to get a damp patch if the roof leaked and it seems unlikely that condensation would cause damp that far from the window". But then suppose I remember that the roof has just been thoroughly checked and I had noticed that the room is prone to condensation in the past. This might well cause me to change my mind about the most likely cause. I cannot make a fair comparison of the alternative explanations unless I take into account both factors -- the likelihood of the causes and the probability of the outcomes given the causes. Notice also that in order to assess the probability of the outcomes given the causes I want to ask questions about "how" -- how could condensation lead to damp in that part of the room?

Can we extend this principle to the case where one of the causes involves design? Absolutely, and it immediately explains our intuitive reaction to the two twin towers scenarios above. So in the case of the two planes hitting the towers we compare the plausibility of it being a double accident to the plausibility that someone planned it. The second explanation wins by an enormous margin. The two collisions are far more likely if someone had planned them and there is a definite possibility that such people might exist and one can imagine an account of how they might have done it. We pretty much dismiss accidental explanations -- even such explanations that might change the likelihood estimate (e.g. a fault in navigation systems that was used on both aircraft) because the awful explanation involving terrorists is all too plausible.

In the second case of the meteors we are much less inclined to accept the design explanation -- even though the likelihood of it being accidental are even smaller -- because the idea of a designer who can control meteor strikes is so wildly implausible. We cannot conceive how they could do it. The probability of the outcome (twin meteor strikes) given the cause (someone who can control where meteors land) is actually very high much higher than the freakish accident. But the likelihood of there being such a cause in the first place are so small that we dismiss that cause and in practice investigate the accidental cause more closely. Note, however, that if someone came up with a vaguely convincing account of how someone might control meteors then we would reconsider.

So it appears we can account for the way we detect design in just the same way we detect any other cause. We assess the probability of the outcome given the proposed cause and we assess the likelihood of the cause existing in the first place. Finally we compare these assessments to alternative explanations.

How does this compare to ID? ID concentrates on just one thing: the improbability of the outcome given that the cause is chance. It does not address the likelihood of the rival causes existing (chance or design). It goes even further than that. It explicitly forbids any exploration of design as a cause because it refuses to be drawn into who or what the designer is or how they implement their design. This means it provides no valid basis for deducing design. Ironically it also means that virtually all the discussion among ID proponents is about the improbability of chance and not the probability of design. They have nothing else to discuss! It is perhaps the greatest irony of ID that its proponents spend most of their time talking about Darwinian evolution. It is one of the biggest testimonies to the power of Darwin's theory that even its greatest opponents cannot conceive of an alternative explanation.