Dr. William Dembski's Explanatory Filter
Dr. William Dembski is a leading intelligent design theorist, and has written several books on the subject, including Intelligent Design: The Bridge Between Science & Theology, and The Design Inference: Eliminating Chance through Small Probabilities (Cambridge Studies in Probability, Induction and Decision Theory). Dr. Dembski has a Ph.D. in mathematics and philosophy, and an M.Div. from Princeton Theological Seminary. One of Dembski's contributions to the intelligent design movement is to set forth one method of detecting design, a method he describes as the explanatory filter.
Dembski describes his three-part explanatory filter together with an example of its use in a paper published at Access Research Network. In the paper, entitled The Explanatory Filter: A three-part filter for understanding how to separate and identify cause from intelligent design,
Dembski found that all formal design detection methodologies can be reduced to three logical steps in what he refers to as an explanatory filter. The filter first asks whether a given pattern is best explained by some chemical or physical necessity or law? If not, can it be explained by chance. If chance and necessity can’t explain the pattern, does it exhibit a “specification” or apparent purpose? If a complex pattern reflecting the integration of numerous stopping points does exhibit purpose and can’t be explained by chance or necessity, then the scientific, logical inference to the best explanation is design.
The steps in this process of elimination can be taken in any order. The question of whether life is or is not designed focuses on ruling out chance and necessity because most biologists acknowledge that living systems appear to be designed for a purpose. They simply claim that the appearance of design is just an illusion that can be explained by chemical and biological evolution, processes driven solely by unintelligent material causes alone. So, the question ultimately becomes whether those unintelligent causes are adequate to that task.
The explanatory filter can be explained using a death investigation by a coroner as an example. Suppose a 98 year old male has been found dead in his bed by the next door neighbor who noticed a noxious smell. The coroner’s check list contains three possibilities: “natural cause” consisting of disease or physical necessity like a heart attack, “accidental cause” like the taking of too many pills from the wrong bottle, or “intentional cause,” like a homicide or suicide. He can also report: there is no “best explanation,” – the data or “clues” are not sufficient to implicate any of the three possibilities. If the coroner finds a knife in the back of the body he may do an autopsy to determine whether death resulted from that stab wound or from an earlier cause with the knife being placed after death to confuse the investigation. If the knife wound caused the death, then it will likely be deemed a “smoking gun” that rules in design and rules out natural or accidental death.
Explanatory Filter Step 1 – Ruling Necessity In or Out
Step one of the explanatory filter asks if some physical or chemical necessity explains the apparent design of a natural object. Consider, for example, a snowflake or salt crystal. Is a snowflake or a salt crystal a design or an occurrence? That is, can a snowflake or salt crystal be explained as being a necessity constrained by known, immutable laws of physics? The answer is yes. Taking a snowflake as an example, the physical properties of hydrogen and oxygen ions that form H2O under certain conditions of temperature and pressure produce intricate hexagonal shapes by known physical laws of thermodynamics. Accordingly, one need not invoke an intelligence cause to explain a snowflake or salt crystal.
However, can physical and chemical necessity explain the particular sequence of genetic symbols of DNA that carry the messages of life? Interestingly, the answer is no. The genetic symbols consisting of nucleotide bases of adenine, guanine, thymine and cytosine (AGTC) can be arranged in any order. Unlike the chemistry of a snowflake or salt crystal, the physical and chemical properties of the four genetic symbols, like dots and dashes in a Morse code message, can hook into any position along the sugar-phosphate backbone of DNA. Just as the letters on this page can be ordered in any sequence, the genetic symbols that specify the letters of life can be arranged in any order to communicate a nearly infinite variety of messages. Watson and Crick predicted this peculiar characteristic, for if the structure of DNA were driven by law, it could not carry the information necessary to generate the seemingly infinite variety of life:
So in building models we would postulate that the sugar-phosphate backbone was very regular, and the order of bases of necessity very irregular. If the base sequences were always the same, all DNA molecules would be identical and there would not exist the variability that must distinguish one gene from another. 
Explanatory Filter Step 2 – Ruling Chance In or Out
Taking the second step of the explanatory filter, if necessity can’t explain a pattern, such as the patterns in DNA, then perhaps chance can explain it. Perhaps random assortments of chemicals came together to produce the first messages of life, for example. After life got started, maybe random mutations in the initial messages would occasionally generate positive functional novelty that would be embraced by the environment. Is 4 billion years enough time for a chance process to turn rocks into intelligent beings that have the capacity to use them for a purpose?
Calculating the odds is not always easy because one cannot always know all the steps necessary to get from point A to point B – to get from no life to life. Darwin could not even look into the cell, much less count the steps, the number of times matter would have to be precisely stopped and started to get life. Also, the possible alternatives at each step are not easy to ascertain. A calculation of probability requires one to divide a particular outcome (e.g., heads in a coin flip), by the number of possible different outcomes (two – heads or tails), times the number of opportunities for the outcome to occur (the number of flips available). If a stadium is filled with 10,000 flippers, we would expect about five flippers to get ten heads in a row, the odds of which are 1 over 2,048. This is because 10,000 times 1/2048 is 4.88. But what if one needed 500 heads in a row, or a very specific sequence of 500 heads and tails or zeros and ones? How many people would one need in the stadium? How many monkeys banging away at typewriters could be expected to generate the first sentence of the Gettysburg address?
The answer to this question reveals a key problem with the chance hypothesis. A sequence that is actually not very long exhausts the universe's available probability resources very quickly. The reason is that probability decreases exponentially as complexity increases only incrementally. Add two digits to the state lottery and no one would ever win.
The exponential decrease is illustrated by a simple example. Suppose you were to put in a brown bag 26 upper case letters of the alphabet, 26 lower case letters and a period, space and a comma. You now have all the possibilities in the bag necessary to write a coherent book. What would be the likelihood of randomly pulling letters in the sequence necessary to spell the first word of the title of the book, which we will title DESIGN vs. Chance. Lets also assume that each time we pull a letter, we will return it to the bag after noting its occurrence? The chance of pulling the D is 1/55. But to get “DE,” the first two letters in the necessary sequence, the chance is 1/55 times 1/55 or 1/3025. To get three letters, DES, the probability is 1/55 x 1/55 x 1/55 = 1/166,375. Four letters, DESI is 1/9,150,625. Five, DESIG is 1/500 million and six, DESIGN is 1/28 billion.
As one can see, chance may explain short sequences having low complexity. However, chance cannot plausibly account for longer and longer sequences where the complexity also exhibits a purpose. The exponential decrease in probability is a major problem for the explanatory power of any chance mechanism, even with a universe trillions of years old. When one does the calculation one finds that even if every elemental particle in the entire universe was a monkey and all monkeys started banging away at typewriters with 55 keys at the rate of 1045 per second at the beginning of the big bang, they still would not have produced a specific sequence equivalent to the first sentence of Lincoln’s Gettysburg Address. Because of the exponential decrease in probability, they would exhaust the available probability resources to get only the first 87 of the 175 letters and spaces that make up the sentence. And the most simple DNA sequence is 100's of times more complex than the first sentence of the Gettysburg Address.
The core challenge to evolutionary theory is to explain the chance occurrence of very long sequences of genetic symbols necessary to generate many integrated functional systems that will survive and replicate in the environment that then exists. The challenge is formidable because the genetic sequences necessary to achieve function are extraordinarily long. Instead of spelling the first six letter word comprising the title, material causes must generate an entire novel to get life started.
Explanatory Filter Step 3 – Identifying purpose – Finding a Specification
The third element of Dembski’s filter is to look for an apparent “specification” or purpose. According to Dembski, the required “specification” is present if it reflects a meaning, structure or function recognizable by a mind that is independent of the significance of the various elements that make up the pattern. For example, the following are two different combinations of six letters of the alphabet: NEDGIS and DESIGN. The first, consisting of NEDGIS, reflects a random ordering of the six letters. It does not reflect any meaning recognizable by a mind that is independent of the significance of each of the symbols that make up the pattern. However, the second sequence that was ordered for a purpose, “DESIGN,” has a meaning that is independent of the six letters in the sequence. The “D” in both sequences has the same significance separately, but in one sequence the relative position of the D enables that sequence to “mean,” as a verb “to intend for a definite purpose.”
In the book by Carl Sagan, Contact, a sequence of prime numbers received by a radio telescope from outer space was deemed to be a possible design or message. In the first step of the filter, the peculiar pattern of beeps and pauses could not be explained by any physical or chemical necessity. Further, its length of over 1000 symbols ruled out chance in the second part of the analysis. The investigation then turned to a search for a “specification,” a meaning or significance for the sequence of prime numbers that was independent of the symbols them selves. They were looking for something that would tie the over 1000 stopping points to an integrated whole. They asked, what meaning lies in a series of prime numbers? Jody Foster, remarked, “Maybe it is an attention getter!” Sure enough the message alerted the SETI researchers to a subsequent message containing the blue prints for building an extraordinary machine that ultimately transported Ms Foster into another world.
Finding a specification or purpose in living systems is not really an issue. Like the SETI researchers in Contact, modern day biologists are trying to find the function or meaning of long sequences of DNA previously thought to be evolutionary “junk.” Parts of DNA clearly code for function or purpose, but what is the meaning of the rest of it? Hence, most biologists concede that living systems give the appearance of design. Recently prominent evolutionary biologists in papers published in the Proceedings of the National Academy of Science acknowledged that the “the challenge for evolutionary biologists is to explain how seemingly well designed features of organism, where the fit of function to biological structure and organization often seems superb, is achieved without a sentient Designer.’”
DNA consists of coded “messages” that are copied by “messenger RNA.” The copy is taken to a processing plant called a ribosome, which then “translates” the message into a functional three-dimensional part or genetic word called a protein. Thus, like the recipes in cook books, sequences of nucleotide bases in DNA carry meaning that is independent of the significance of each of the symbols that make the “message.” They not only look like specifications, they function as specifications.
“Smoking Guns” of intelligence
Albert Harrison, an expert on detecting design from aliens identified what he believed would amount to a smoking gun that would clearly implicate the existence of alien intelligence. If we could find a symbol sequence that did any of the following we could be certain that it was generated by a mind for a purpose:
“An elusive mathematical proof, the solution to a scientific problem, the blueprint for a ‘miraculous’ invention.”
Curiously, the very message Harrison is looking for in outer space, literally sits right beneath his nose - in each of the cells that comprise it. Each cell contains in the messages in DNA not only “the blue print for a ’miraculous’ invention,” but the operating instructions, routines and biological clocks necessary to assemble it, maintain it, operate it, replicate it and then even to kill it. A similar invention was present in the simplest and most ancient form of life, a bacterium. The smoking gun that Darwin’s material causes need to explain away are the messages in DNA.
- James Watson, The Double Helix: A Personal Account of the Discovery of THE STRUCTURE OF DNA, (Touchstone 1968) p 52-4.