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The Scientific Status of Intelligent Design

by Thomas B. Fowler


This essay by Thomas B. Fowler shows where intelligent design fits in among other theories of human origins and evolution, considers its scientific status, and indicates what is necessary to prove its hypotheses.

Larger Work

Faith & Reason


503 – 538

Publisher & Date

Christendom Press, Front Royal, VA, Winter 2006


The intelligent design movement has been growing in strength over the past decade, and is now very influential in the evolution debate. It has been especially strong in faith-based communities; recent comments by Pope Benedict and Cardinal Christoph Schönborn have given Intelligent Design some degree of credibility among Catholic Christians.1 Such support is leading many to the suspicion that Intelligent Design is just a Creationist Trojan Horse trying to break into the realm of legitimate science. Because of its increasing stature, Intelligent Design is at the forefront of many scholarly and legal battles, as well as the culture wars raging in the United States. Most criticism of the Intelligent Design school characterizes it as unscientific because it relies on notions such as that of a "Designer," thus violating the requirement to restrict science to naturalistic explanations. In some few cases, ID arguments are considered on their own merits.

Unquestionably, concern for the integrity of science is legitimate: science is the product of an enormous amount of highly disciplined creative work over many centuries, and it forms the basis of much of our modern industrial society. Science cannot fall hostage to any group or ideology; its success and well-deserved prestige depend upon maintaining the highest standards of objectivity and methodology in its quest for explanations of phenomena. So is Intelligent Design a threat to science? Does it seek to pervert the foundation of science? Can it make any real scientific contribution? To answer these questions, and clear the air, an objective analysis of the scientific status of Intelligent Design is needed with respect to bona fide scientific issues, naturalism, and the history of science. That is the purpose of this article; the article does not seek to convince the reader of the truth or falsity of Intelligent Design, to determine whether it is indeed true, or to debate the merits of its metaphysical or theological implications.

This would appear to be an easy task, but it is not. Nearly all discussions of Intelligent Design come from sources that have a vested interest in the matter, either pro or con. This lack of objectivity makes it difficult for readers unfamiliar with the relevant biology, mathematics, and other disciplines to reach an informed conclusion on the matter. Worse, the discussions tend to be very confused, and there is often a willful misunderstanding of Intelligent Design, a deliberate attempt to characterize it as something that it is not. This strategy has the clear goal of discrediting Intelligent Design so as to avoid confronting its challenges directly. The discussion here will enable the reader to evaluate the scientific status of Intelligent Design in an impartial and well-grounded manner.

To aid our evaluation of Intelligent Design, we shall first review briefly the modern history of the evolution controversy, and position of Intelligent Design within it.2 This will allow the reader to understand the landscape of the evolution controversy, and how Intelligent Design relates to the other major schools of thought about evolution. There are four such schools: Neo-Darwinism, Creationism, Meta-Darwinism, and Intelligent Design.

Recent History of the Evolution Controversy: Why the Intelligent Design School Arose

Darwin published his great work, Origin of Species, in 1859. There he propounded the notion of organic evolution, according to which all life forms arose by variations from a primitive ancestor. Darwin's key contribution was not the idea of organic evolution, but rather the naturalistic mechanisms that could make it work-natural selection together with genetic variation.3 In Darwin's formula, the mechanisms give rise to common descent with modifications: a primordial living organism arose and replicated. Its descendants with slightly better (or much better) characteristics were able to leave more descendants than others (survival of the fittest). In this way there was a gradual improvement in the population, and eventually the improvements led to new species and higher taxa. In this manner, there was a gradual increase of complexity in flora and fauna, entirely mediated by natural processes, leading to the generation of all life forms. Though problems with the theory were quickly uncovered, it gradually won over the scientific community. However, by the early 20th century, many difficulties had accumulated, and new scientific results in the area of genetics were growing. An overhaul of the theory was therefore required. This was done in the first half of the 20th century, and work has continued to the present day.

This new theory, or New Synthesis — more commonly known as Neo-Darwinism — is the school that now dominates the scientific landscape. Despite a number of serious theoretical and experimental shortcomings, this theory has the allegiance of the majority of scientists, educators, and educated people. In this theory, the drivers of evolution are natural selection and random mutation, which according to the theory work together to create and incorporate new genetic information, leading to new species and ultimately higher taxa, and thus all of the diversity that we see today in the biological world. An immediate inference is metaphysical naturalism: the belief that all biological entities and phenomena arose through natural means and can therefore be explained by science.4 It is important to note that all schools of thought about evolution accept the existence of these two mechanisms (natural selection and random mutation); what is disputed is their efficacy in bringing about new biological entities, and of course the inference about naturalism. Specifically, all schools agree that the mechanisms can account for microevolution (small adjustments in characteristics of a population); they disagree about the degree to which the mechanisms can account for macroevolution (creation of new, more advanced types of organisms). Though much of the evidence for Darwinian evolution is perforce historical or circumstantial, and relies on inference rather than direct demonstration, its proponents believe that all of its problems will sooner or later be resolved, and that the evidence for the theory is so overwhelming that no serious questioning of it is possible.

Not everyone who is knowledgeable about the subject shares this view. Over the last forty years, the Neo-Darwinian school has increasingly found itself under attack from both scientists and non-scientists alike, especially over the idea that its proposed engine can account for all life.5 Nonetheless the Neo-Darwinian school has maintained its hegemony in the academic and intellectual realms, and has found the courts sympathetic to its message that dissenting opinions should be suppressed, at least in the classroom.

Creationist Backlash

By the early 1960s, the materialistic and frankly anti-theistic implications of Neo-Darwinism were setting off alarm bells in the evangelical community. Coupled with a commitment to some form of Biblical literalism, this led to a resurgence of Young-Earth Creationism (belief in recent creation of the earth, less than 10,000 years ago), the second major school in the evolution debates. Most early Creationist efforts tended to be rather amateurish, but over the years quality has grown steadily. With increasing numbers of converts, especially among scientists, some modern Creationist research and analysis is fairly sophisticated, if limited in quantity. Of course, for the Neo-Darwinians and most other scientists, the whole idea is still laughable, and no amount of sophistication can change that. Ridicule has never daunted the Creationists, however, and they began preaching their message in many venues, including schools and colleges, where they challenged evolutionists to debate the issues. They found sympathetic audiences in many places and by the 1980s had became formidable advocates, winning over many and often defeating opponents, who seemed confused and bewildered by their arguments. This, understandably, alarmed evolutionists, as did their efforts to win a place for their views in public schools and textbooks. Those efforts have triggered an on-going series of legal and legislative battles. To counter the growing efforts of Creationists intent upon getting a hearing for their views in public schools, the National Center for Science Education (NCSE) was founded in 1981, and prominent scientists wrote books to refute Creationism.6 The debate became more acrimonious with the publication of provocative works by Richard Dawkins and others, including Dawkins' famous Blind Watchmaker (1986), which openly promoted the thesis that evolution implied atheism.7

The Meta-Darwinian School

Serious trouble for Neo-Darwinian theory was brewing on other fronts as well. In 1966 a famous conference was held at the Wistar Institute in Philadelphia. The title of the conference was "Mathematical Challenges to the Neo-Darwinian Theory of Evolution." It was an outgrowth of an informal meeting at the house of physicist Victor Weiskopf and was chaired by biologist and Nobel laureate Sir Peter Medawar. The general thrust of the conference was that evolution is impossible if it must rely upon the mechanisms proposed by Neo-Darwinism, though by no means did all of the participants accept that conclusion.

There was also growing concern among biologists that explanations of matters such as discontinuities in the fossil record were not forthcoming from the Neo-Darwinian school. This situation was embarrassing and seemed to be supplying Creationists with much ammunition. In response Steven J. Gould and Niles Eldridge formulated a new version of evolutionary theory, which they christened "Punctuated Equilibrium." This theory proposed that evolution happens in spurts, not continually and gradually, as demanded by the Neo-Darwinian model. Gould and Eldridge stirred up much opposition, and many felt that a summit conference was needed to deal with some crucial issues.

As a result, a conference was held at the Field Museum in Chicago in October of 1980, attended by the major figures in evolutionary biology. The central question addressed was the validity of extrapolation from micro- to macroevolution. There were considerable fireworks at the conference; even Science magazine conceded that most felt the answer was "No". But by this time the influence of Creationism had become such a threat that, in an unprecedented move, no formal record was made of the conference proceedings, so as not to give Creationists any more ammunition.

So by the early 1980s, science had amassed a huge amount of circumstantial evidence pointing to evolution, but some stubborn problems persisted.8 Hence scientists found themselves staring down a wrenching dilemma. On the one hand, many felt that the reigning Neo-Darwinian theory was seriously deficient, and needed work. However, they did not want to abandon the idea of naturalistic evolution. On the other hand, they felt (correctly) that any perceived crack in the scientific position, any suggestion that there was not monolithic agreement about evolution, would be ruthlessly exploited by Creationists. As most regarded the latter as the worse of the two alternatives, they tended to close ranks around the Neo-Darwinian theory, despite its admitted problems.

The unsatisfactory nature of the situation bred two new schools. Some scientists, including Stuart Kauffman and Lynn Margulis, proposed new mechanisms for evolution. Kauffman emphasized the self-organizing qualities of matter as a way to deal with the long-standing problem of astronomically improbable events required by the orthodox theory. Margulis sought to defuse the problem by arguing that complex organisms can be built via symbiosis. Another dissenter, physicist and astronomer Fred Hoyle, even proposed that microscopic life originated elsewhere in the universe, where conditions might be more favorable. Their work, together with that of Gould and Eldridge and others constitute an eclectic cadre of theories forming the Meta-Darwinian school, the third major school in the evolution debate. This school accepts that life forms came about through naturalistic means, but believes that mechanisms beyond those of the Neo-Darwinian school are required to account for it. For individual biologists, the boundary between it and the Neo-Darwinian school is often not hard-and-fast.

The Intelligent Design School

By the late eighties, an assorted group of thinkers had become quite dissatisfied with Neo-Darwinism. These thinkers were particularly uncomfortable with the way some involved in the debate were using it to promote philosophical and anti-religious agendas. Though they drew on many of the same concerns and arguments as the Meta-Darwinian school (and to some extent the Creationists), they felt that a new approach was needed because of disagreements with these two schools. In particular, this group rejected the purely naturalistic outlook of the Meta-Darwinian school, but did not wish to embrace Creationist views on the young age of the earth, with its attendant implications for geology, physics, astronomy, and other sciences. Spearheading this group was Philip Johnson, a law professor who had studied evolution in depth. Johnson published several very influential books in the early 90s, beginning with Darwin on Trial (1991).9 This book may be considered to mark the beginning of a new movement referred to by its members as the Intelligent Design school, though its roots can be traced back to work by Michael Denton, Norman Macbeth, the Wistar Conference, Harold Yockey, and perhaps even to the work of longtime critic of Neo-Darwinism Fred Hoyle.10 Intelligent Design is the fourth and youngest of the four major schools of thought about evolution today.

Johnson's books, however, were restricted to criticism of Neo-Darwinism, and presented little in the way of positive scientific contribution. So the defining event of the decade for this school was undoubtedly the publication of biochemist Michael Behe's Darwin's Black Box (1996), which challenged Neo-Darwinian theories about the emergence of what everyone acknowledged as extremely complex structures and systems. Specifically, Behe argued that there are at least some biological systems so complex that random or chance processes at the heart of Neo-Darwinian theory cannot account for them. Such systems are characterized by what he termed irreducible complexity. Behe's work was later taken up and refined by William Dembski and others. By the early 2000s, the Discovery Institute in Seattle had become a major focus for Intelligent Design activity and the Intelligent Design school had become a formidable participant in the debate, if quite small compared to the others. In the last few years, Intelligent Design has supplanted Creationism as the vehicle for introducing challenges to Neo-Darwinism in public school science courses, so it has become embroiled in many court battles. As its visibility has increased, attacks on its members by the Neo-Darwinian school have also increased.11

So the current situation may be summarized as follows. There are four competing schools of thought about evolution. The principal divide is between those that believe natural mechanisms alone are sufficient to account for all life on earth (the dominant Neo-Darwinian school and the much smaller Meta-Darwinian school) and those that reject this belief (the Creationist school and the Intelligent Design school). The Neo-Darwinian school argues the sufficiency of random mutation and natural selection acting together to create the "common descent with modifications" paradigm. The Meta-Darwinians agree that natural mechanisms alone are sufficient to account for all life, but reject the "one size fits all" approach of the Neo-Darwinian school, arguing that the mechanisms proposed by the Neo-Darwinian school are inadequate for the job, and are capable of explaining only a fraction of life's history. Another great divide is the age of the earth. Creationists dispute the old age claimed for the earth, and thus most of the conclusions of the Neo-Darwinian school about how life changed, including common descent from a single ancestor.12 The Intelligent Design school accepts an old age for the earth, and all of the science behind it, and accepts much of common descent; however, it rejects the sufficiency of natural mechanisms to account for all of the changes required for the history and development of life on earth.

Figure 1 shows the four schools and their relationship. In terms of numbers and adherents, the Neo-Darwinian school has the allegiance of most scientists, educators, and college-trained professionals. The Meta-Darwinian school is very small but growing among those same groups. Creationism has made great inroads in the general public, where it far outweighs the other schools. Intelligent Design is still rather small in numbers but is gaining support among all groups.

Summary of Principal Ideas of the Intelligent Design School

Behe's notion of irreducible complexity is based on a key fact of many complex systems, namely that they have at their core a set of interacting elements all of which must be present and functional for the system to work at all. Thus in the case of a watch, there must be a mainspring, appropriate gears to drive the hands, an escapement mechanism, and numerous other components. If any of these is missing or nonfunctional, the watch will not keep time. It is important to note that in such case, the watch does not retain some of its ability to keep time; it loses all of its ability to do so. This broken configuration serves no useful function and must either be repaired or discarded.13 Behe himself uses the example of a mousetrap.14 This situation, in which no component can be removed without destroying the functionality of the overall system or device, is irreducible complexity.

The implication of irreducible complexity for biological systems is straightforward: such systems could not have originated through any incremental, step-by-step process, because all of the components must be present for the system to work; there is no partial functionality. Such an incremental change pattern is at the heart of Neo-Darwinian theory, since that theory has no alternative to create structures and systems except by small, incremental changes. Hence the irreducibly complex structures which Behe claims are common in biological systems cannot have originated as Neo-Darwinism claims, and therefore the theory must be false.15 This is a strictly scientific argument, albeit one which rests on the contested notion of irreducible complexity. Clearly the argument is sound only if Behe can demonstrate that some biological systems are, in fact, irreducibly complex. More important, perhaps, is the implication of irreducible complexity for a deeper view of biological systems: irreducible complexity effectively partitions biological systems (and structures) into disjoint classes or groups. The implications of this — which is really at the heart of scientific theory in Intelligent Design — will be discussed below.

While many complex systems, especially man-made machines, exhibit irreducible complexity, it is clear that not all complex systems do so. Political and economic systems, for example, generally do not. Thus the economy would not collapse if General Motors (or any other company) failed; nor would the government collapse if the president were killed. Furthermore, these types of systems evolve over time, growing more complex in a gradual fashion that does not appear to be designed.16 Accordingly, critics of Intelligent Design have sought to show that biological systems and structures are not irreducibly complex and can in fact originate by small steps without the need for simultaneous creation of multiple complex structures — something that even they concede would be impossible.

William Dembski has elaborated Behe's ideas, and formulated an algorithm, called the Design Filter, to ferret out cases of design from cases of chance occurrence. Actually, design is an inference from the filter; what the filter actually does is partition events into one of four categories. The net result of the filter is Dembski's Law of Small Probability: "specified events of small probability do not occur by chance."17 To understand this law, we must examine the Design Filter, which is shown in Figure 2.18

The filter starts with an arbitrary event, and then analyzes it to determine into which of three classes it falls. If the event has a high probability of occurring, such as water turning into ice when the temperature falls below 0° C, or a stone falling to the ground when dropped, it is something that happens according to a regular pattern most likely governed by a natural law. Thus it is a case of regularity. For the second stage of the filter, if the event has moderate probability of occurring, but not a high probability, it is attributed to chance. An example would be that of flipping 10 straight heads with a fair coin. This is not a high probability event, obviously, as it only occurs in 1 out of 210 = 1024 times, but it could occur. If the event has a very small probability, it could still be due to chance. The third stage of the filter examines a case crucial to the Intelligent Design argument. Here the question asked is whether the event or subject conforms to a particular pattern, one that can be ascertained or recognized independently of the event itself. An example of this case is Mt. Rushmore. Clearly, finding a pattern such as that of a rock wall containing the faces of four presidents is very unusual, i.e., very small probability. But the pattern is recognizable independently of the rock wall. This is what Dembski calls a specified event. A specified event of small probability must, in his view, be attributed to design. On the other hand, if we consider the rubble underneath the faces in the rock wall, the pattern of the rocks is unique — it is quite unlikely that we could find an identical pattern of rock rubble anywhere else in the world. But unlike the presidents, the shape of this rubble does not conform to any pattern recognizable independently of the rubble itself. Therefore this pattern is not specified, and its occurrence, though of small probability, must be attributed to chance. Note that like Behe's irreducible complexity, Dembski's specified events of small probability effectively partition the space of biological systems and structures into disjoint groups.

The relationship between Behe's irreducible complexity and Dembski's Design Filter is not difficult to understand. Dembski's filter is designed to ferret out cases of specified events of small probability Such events cannot reasonably be attributed to chance, according to the Intelligent Design school. Irreducibly complex biological systems are specified on account of the unique function they perform, such as the locomotion of a cilium. They are of small probability because component parts must have exactly the right structure; slight deviations render the system inoperable.

Empirical questions that turn on very small probabilities can be difficult to investigate in the laboratory, however, because of the time constraints on laboratory experiments, which must be completed in months or years perhaps, but not millennia or eons. For this reason, most discussion of the biological issues has been based on theoretical arguments, utilizing mathematical formulations. Clearly questions about feasibility can be investigated from a theoretical point of view if the theory can be adequately anchored in biological realities. That is to say, one must show that any theoretical discussion of the possibility or probability of biological change combines rigorous mathematics with empirically justified premises. This is important, because many discussions center on calculations of the probability that such-and-such a protein or other biological molecule or structure could arise through chance mutations. Unfortunately, such calculations require many assumptions about pre-existing products, pathways, environmental factors, and boundary conditions, all of which are extremely difficult to verify because they refer to events and situations in the past that cannot be replicated or even known.

Still, at first glance one might assume that a brute force approach is adequate on evolutionary time scales. There is plenty of time for the needed components to arise by chance, and thus the Design Filter would fail, as would Behe's argument about irreducible complexity. Unfortunately, the brute force approach — sampling all possible forms of a given protein — is infeasible because of the combinatorial explosion associated with proteins of even moderate length.19 The protein cytochrome c is often used as an example, because there are a large number of variants of it, and it occurs in organisms throughout the taxonomic scale. Generously allowing for variants, Yockey has determined that for every functional sequence, it would be necessary to generate (code for) and then test 5 x 1043 nonfunctional sequences — an absurdly large number when we recall that the age of the earth is only about 1017 seconds.20 Obviously, the information for cytochrome c was not determined in this manner. The problem is even more serious for proteins that do not allow of variation, because they are all-or-nothing, such as histone-4, which functions in gene activation and chromatin assembly.21 Unless histone-4 is exactly right (with only variants in the encoding DNA permitted by the redundancy of the code), cells would be unable to divide properly, or turn on and off genes properly, and the organism would perish. There are more than 200 base pairs in histone-4 that are conserved across all of biology. How was the information in the sequence generated? Once again, not by brute force search: the number of possible 200 base-pair sequences is 4200 = 10120 To get a feel for how large this number is, note that high estimates of the number of atoms in the universe are about 1087. The age of the universe is about 1018 seconds. This means that if every particle could have tested one sequence per second since the beginning of the universe (an obvious impossibility), only 1087 x 1018 or 10105 sequences could have been assayed — a number 15 orders of magnitude less than the number of 200 base pair sequences.

The only possible escape from this combinatorial black hole is to postulate that the proteins evolved from simpler forms with some functionality, so that there was a path through the space of all possible sequences that rendered testing of all of them unnecessary. The problem is particularly acute in the case of histone-4, because the histone-4 had to co-evolve with the entire chromosome system, which presumably developed from primitive asexual mechanisms. There are, however, serious difficulties with this approach, because of the need for simultaneous changes in multiple protein lines as the hypothetical co-evolution progressed. In fact, it generates an even worse combinatorial problem. There are, in addition, other problems related to the ability of primitive systems without sexuality and crossover (which histone-4 controls) to evolve at all.22 Furthermore, it is important to realize that the hypothetical evolutionary path would have to reduce the number of sequences to be assayed by roughly 100 orders of magnitude — the difficulty of which can scarcely be overstated.

These types of combinatorial arguments are a mainstay of all three schools challenging Neo-Darwinism. They are not restricted to the Intelligent Design school. The point of the arguments is to establish that the chance mechanisms at the heart of Neo-Darwinism cannot possibly have produced the complexity exhibited by even the simplest biological creature. In the case of the Intelligent Design school, this argument is then used to show that the specified biological systems are of very low probability. Of course, as noted above, the Neo-Darwinian school counters that the structures and systems in question did in fact arise from similar predecessors, so that the probability of their originating by chance is much greater than the calculations suggest. At present the matter is unresolved.

The Scientific Status of Intelligent Design

So is Intelligent Design scientific? That is, can we regard Intelligent Design as a legitimate scientific theory to challenge Neo-Darwinism? It is clear that Intelligent Design asks important scientific questions about the limits of physical processes to generate complex structures and processes by random means. Intelligent Design is not intended as a complete replacement for Neo-Darwinism; as we have already discussed, Intelligent Design accepts the basic mechanisms of natural selection and random mutation. It only challenges the inference (some would say long-range extrapolation) that these mechanisms can explain all biological entities. What Intelligent Design would do is, in effect, to partition biological changes into disjoint ordered groups — islands if one likes — within which transformations are possible by natural means. Between any two such groups, change mediated by natural processes is so improbable as to be effectively impossible.23 Neo-Darwinism, of course, denies that there are any such unbridgeable gaps. The Meta-Darwinian school agrees that there are gaps, but believes that other natural mechanisms can cover them.

To better understand the difference between what the Intelligent Design school is proposing and what Neo-Darwinism maintains, we need to examine the core argument of the Neo-Darwinian school in more detail. Evolutionists agree that arbitrary combinations of structures and systems are not viable and that not just any biological entity can be transformed into any other. One cannot transform a dog into a cow, for example; but the two have a common ancestor, and so are connected in that way. They believe that there are sufficient paths available so that all the transformations required to generate all life forms (living or extinct) are possible. By "possible" here they mean not just physically possible, but capable of occurring with a sufficiently high probability for us to assume that the transformations did in fact occur. Thus, any structure that appears to be designed can in fact arise through naturally occurring transformations. Consider the following remarks from a respected graduate textbook on evolution by Mark Ridley:

Evolutionary biologists are sometimes challenged with arguments about functionless rudimentary stages or the impossibility of complex adaptive evolution. It is impossible to imagine, someone will insist, how a certain character could have evolved in small, advantageous steps. In reply, the evolutionary biologist may offer a possible series of stages by which the character might have evolved. It is important to bear in mind the status of the evolutionary biologist's argument here. In some cases, the series of stages may not be particularly plausible or well supported by evidence. The argument, however, was put forward solely to refute the suggestion that we cannot imagine how the character could have evolved [italics added].24

And further:

Such speculations are not intended as prize specimens of evolutionary analysis. The series is not claimed to be particularly profound or realistic, or even very probable. The long evolutionary history that precedes any complex modern adaptation will appear, with hindsight, to be an improbable series of accidents; the same point is as true for human history as evolution. Given the state of our knowledge at any one time, we can reconstruct the evolutionary stages for some characters with some rigor . . ., but not for others. For these cases, we can only guess at the possibilities because we cannot conduct a rigorous scientific investigation [italics added].25

This gets a bit off the track, since the whole point of the discussion is that the transformations in question needs some pathway that has a "reasonable" probability of occurring, such that when many are strung together, the entire sequence has a probability whose exponent is not measured in negative thousands (e.g., 10-5000). Recall that the thrust of the critics' objection is that no such pathways exist. That objection will not be met by guesses and speculation.

Ridley continues:

It is fair to conclude that there are no known adaptations that definitively could not have evolved by natural selection. In other words, we can conclude that all known adaptations are, in principle, explicable by natural selection.26

Paraphrasing slightly, we have, in effect, the following argument:

We can imagine a series of steps to make any transformation, therefore we conclude that anything could have evolved.

Or even more succinctly,

We can imagine it, therefore it happened.

The soundness of this argument is not the subject of the present investigation. We are interested in what it reveals about the commitment of Neo-Darwinism to an amorphous deep structure of the space of biological components and systems. What it clearly shows is the exact opposite point of view on the possibility of transformations from that of the intelligent Design school. That is, the Neo-Darwinian position postulates that no set of disconnected islands exist, among which transitions are impossible. The Neo-Darwinian school does not maintain that arbitrary transformations are possible, e.g., turning a dog into a cat. But it does maintain that any structure or system can, in a finite number of steps of finite probability, be reached from any other, or from some predecessor common to them both.27

The situation is illustrated in a highly schematic form in Figure 3, which shows the views of the two schools for the case of a hypothetical transformation from point A to point B and then to point C.28 The Neo-Darwinian school envisions the stable entities as falling into overlapping groups, so that a path which can be traversed in finite time exists between the two points, as shown. For example, this may by a transformation of one structural protein to another. Note that the path from A to B via D is not envisioned as possible. For the Intelligent Design school, on the other hand, there are gaps between groups of entities, within which transformations are possible. Thus, getting from A to B is possible, but not from A to C.

If Intelligent Design is correct, it should become possible to map these gaps and to understand them from the theoretical standpoint of physical chemistry. Such an investigation would likely involve a great deal of higher mathematics; it would have to examine biological structures in some very high dimensional spaces and investigate the effect of physical laws on their possible transformation in those spaces. The quantitative methodology to analyze complex structures and discern those that cannot have arisen by random processes is only the first step in building the new theory of possible biological transformations. In effect, Intelligent Design would reveal a deeper level of structural characteristics of biological systems and entities than is now recognized. If brought to fruition, this would have an impact not only on the study of evolution, but on much modern-day research, because it would show that certain types of change cannot occur, while other types may be quite likely. The impact in areas such development of antibiotics and antiviral drugs would be significant. For example, if a drug could so change the environment of a pathogen that no feasible random change to its genome could allow it to survive in that environment, then a potent new treatment method would become available.

Contrary to popular belief, Intelligent Design does not start with the idea of a "Designer." The "Designer" is an extra-scientific inference from a particular scientific conclusion about the impossibility of certain physical transformations. In the same way, atheism is an extra-scientific inference from Neo-Darwinian theory drawn by many of its advocates, including Dawkins, Dennett, and Provine, who believe that the physical transformations in question are possible.29

So it is an empirical question whether the mechanisms of Neo-Darwinian theory can create the type and quantity of new biological information required to explain the origin of all life forms observed now and in the fossil record. This is important to point out, no matter how self-evident it may be, because both sides in the dispute seem to think that they can dictate an answer — namely, the answer they would like to have, generally based on intuition. But what the Intelligent Design school is doing is definitely not to surreptiously introduce religion into science. The proponents of the school are asking an empirical question which should be of great interest to all scientists.

Unfortunately, Intelligent Design proponents sometimes speak in terms that might suggest knowledge of design and perhaps of a designer should become part of science, rather than being an inference from it. Thus an assertion such as, "If living things are designed, then it is a mistake to assume that they are not," can easily be construed to mean that design and a designer should become part of biology30 Worse, at times Intelligent Design advocates discuss miracles in connection with science, as if science has to be able to explain miracles.31 This is a very confused position. Science will never be able to explain miracles. If it could explain them, they would not be miracles.

The fact of the matter is that even if a scientist knows that something is designed, that knowledge will have very little bearing on what he does when he is trying to figure out how it works. Think, for example, of a scientist trouble shooting a computer program. The scientist's job is to go where the facts lead. If something has been designed, it will only be possible to explain part of its origin in naturalistic terms. That will come out in the wash, so to speak. The main requirement for good science is honesty and humility in the face of experience and observation. The case of the universe as a whole is another example. Even if an astronomer believes that the universe has been designed, his work is not changed: he will still be interested in measurement of distances, stellar and galactic evolution, quasars, fields, and the other subjects currently of interest to astronomers. He may marvel at the wonders of creation as revealed by the cosmos — many enter astronomy for just this reason — but that will not affect his work as a scientist.

Some have attempted to make Intelligent Design a separate scientific theory or explanation.32 Undoubtedly, the notions of irreducible complexity and specified events of small probability are legitimate scientific hypotheses and can generate avenues of scientific investigation. This is because they make testable, empirical claims about the nature of biological systems. If the claims are true, they will tell us much about nature, from a strictly physical standpoint. We may also be able to make important extra-scientific inferences from them. But considered in themselves, they are not, in any sense, "Intelligent Design science" or anything of the sort. It is extremely misleading to characterize the Intelligent Design movement in this way. Doing so only gives its critics legitimate ammunition to accuse it of subverting science.

Indeed, there is no "Intelligent Design Science," or "Creation Science," or for that matter, "Neo-Darwinian Science." There is only science — good science and bad science, perhaps, but only science. Science is the pursuit of truth about nature.33 There cannot be two truths or Averroistic dual truth — there can only be two competing theories. One will eventually win out, or both will be overthrown. The competing theories can advocate different research programs, and be looking for different phenomena. Sooner or later, the predictions of one will be vindicated, and those of the other will not. Intelligent Design proponents are right to seek out places where their theory of disjoint classes of systems can be tested and verified, just as Creationists are right to look for those places where their theory of rapid, high-energy processes can be verified.34 No one should be afraid of this competition, nor try to block it. Indeed, such challenges should be welcomed by any reigning theory, as they give the theory a wonderful opportunity to prove its superiority — if indeed it is the best theory.

It is important to note, however, that at present, Intelligent Design is quite far from establishing its claims about irreducible complexity or the impossibility of the natural occurrence of specified events of small probability. The methods employed so far utilize human judgment and often rely upon combinatorial arguments, which unfortunately require many assumptions that are difficult to verify. Much further work needs to be carried out in order to establish the central scientific tenets of Intelligent Design. The sequence of Intelligent Design reasoning and demonstration may proceed as shown in Figure 4. As shown, the Intelligent Design school is less than halfway to its goal.

Naturalism and Other Criticisms of Intelligent Design

First let us deal with a common but flagrant propaganda argument. Critics often assimilate Intelligent Design into Young Earth Creationism and place both schools in the same camp as believers in the Flat Earth.35 This is a very serious error. Not only is it a material misrepresentation — willful in this author's opinion — but it betrays a profound ignorance of the issues. First, no educated person since the time of the ancient Greeks has believed that the earth is flat.36 Such a belief would have been quite incompatible with even the Ptolemaic theory, which advocated a geocentric view of the universe in which the planets and stars orbited the earth on spheres. Obviously, it is incompatible with the heliocentric theory. Neither Creationists nor Intelligent Design advocates nor anyone else involved in the evolution debate today has anything in common with Flat Earth or other long-outmoded ideas. Creationists do dispute the old age of the earth, though they seek to do so by means of scientific arguments. The validity of their case is beyond the scope of this article.37 Second, most in the Intelligent Design camp reject the young earth hypothesis and center their case on the possibility of certain types of physical transformations. While Creationists also argue along these lines at times, it is not their main focus.

Criticisms based on naturalism are a bit more sophisticated. But the issue of naturalism, like many others in the evolution controversy, is confused because the word "naturalism" has multiple meanings, and these are often confused or conflated.38 If "naturalism" is taken in the methodological sense, it means that science is restricted to explanations and theories that utilize natural laws and experiments that are publicly repeatable. It cannot invoke spiritual forces, unobservable mental phenomena, or any other non-natural occurrences. As we have already seen, Intelligent Design fully subscribes to methodological naturalism and does not (or should not) propose that science begin utilizing non-natural forces or laws. Methodological naturalism says nothing about what the scientist will find in his quest; this should, however, include openness to discovering certain limits to knowledge. The scientist looks for laws and regularities in nature and formulates hypotheses based on what he actually observes, not on what he would like to observe. If there are deep structural partitions among biological entities that affect the possibility of transitions among them, that is just the way nature is; and it is no more or less natural than the limits imposed by the maximum speed in the universe (the speed of light), the need for entropy to increase, or the Uncertainty Principle. While scientific hypotheses and laws exclude the use of mechanisms not directly observable, such as spiritual forces, they do not require any assumption about the degree to which observable phenomena can be explained. Indeed, any such assumption would be an external interference with the operation of science. The laws of nature, and their ability to account for phenomena, are what they are, not what we would like them to be.

But there is another sense of naturalism, metaphysical naturalism, which goes well beyond science in the strict sense. It attempts to use science to support a particular metaphysical (or philosophical or theological) position. Generally, the position is that all natural phenomena must be explainable by the methods of science, and no gaps in our knowledge should remain to be filled with extra-scientific explanations (such as an Intelligent Designer). This is because a metaphysical position has been staked out, generally that there is nothing other than nature in the sense of what can be examined by science. So the metaphysical naturalist argues along these lines:

Naturalism [i.e., the ability to explain all natural phenomena with science] is what has always characterized science. To give up naturalism is to abandon science altogether.

We have been down this road many times before: a metaphysical principle is grafted onto science — usually to support some extra-scientific belief — and the resulting conglomeration is taken as indispensable to science, if not to all of human knowledge. Denial of the metaphysical principle then becomes tantamount to denial of science itself. The example of Kant immediately comes to mind. For Kant, Newtonian physics, with its absolute space and time and Euclidean geometric foundation, was the way we had to think about the world; any other type of science was inconceivable. Subsequent developments in physics and mathematics destroyed this position. When the idea of the Big Bang and a finite age of the universe was new, physical chemist Walther Nernst objected on the grounds that "infinite duration of time was a basic element of all scientific thought, and to deny this would mean to betray the very foundation of science."39 Of course, it did not do so. Similarly, when quantum mechanics was new, many including Planck, Einstein, and de Broglie argued against it on similar "if it isn't my metaphysics, it isn't science" grounds. As Xavier Zubiri has observed:

Uncertainty or indeterminism seems to be what is most opposed to the character of all scientific thinking. Planck, therefore, indignantly rejects this concept; to renounce determinism would be to renounce causality, and with it, everything that has constituted the meaning of science from Galileo up to the present day.40

But they too had mixed metaphysics with science:

That renunciation [of deterministic physics] was for the representatives of the old physics the great scandal of the epoch. How is it possible for physics to renounce explanation of the origin of all movement? This heroic renunciation, nonetheless, engendered modern physics.41

So it is extremely important, though at times very difficult, to separate science from metaphysics because of the desire to make science support a particular worldview.

Of course, there is nothing wrong with analyzing science and looking for metaphysical or theological implications. Indeed, this is a very important task, because it helps us to understand the position of science in the context of human knowledge, to evaluate some philosophical and theological claims, and to determine the limits of scientific knowledge. People have in fact been doing this for centuries. What is wrong is any attempt to go in the opposite direction and force science into some arbitrary mold by saying that the scientist has to discover certain things — be it that the universe is infinitely old, or that space is absolute, or that Darwinian mechanisms can account for all life forms — and that anyone who disputes these things cannot be doing bona fide science.42 This attitude leads naturally to a "facts are irrelevant" mentality, exemplified well by Richard Dawkins:

Darwin's theory is now supported by all the available relevant evidence, and its truth is not doubted by any serious biologist . . . I suggest that it may be possible to show that, regardless of evidence, Darwinian natural selection is the only force we know that could, in principle, do the job of explaining the existence of organized and adaptive complexity.43 [italics added]

So if the Intelligent Design school is proposing that some transitions are impossible, we can expect to hear arguments that accuse it of trying to destroy science. Just as in the other cases, such arguments reveal the metaphysical agenda of the objectors, rather than anything fundamental about science.

It is perhaps unfortunate that Intelligent Design does not more clearly separate its purely scientific hypotheses and theories from those extra-scientific inferences that have become the focus of so much discussion of it, both pro and con. In no case, however, should Intelligent Design advocates speak in terms that can be (mis)interpreted to mean that it is introducing non-naturalistic causes or forces into science. The thrust of its scientific hypotheses has to do with the limits that may exist on physical transitions in virtue of natural laws. These limits in turn imply limits to scientific knowledge. The existence (or non-existence) of these limits is an extremely important and engaging scientific question, and it should not be obscured by metaphysical controversies that are not germane to it.

And what does the history of science tell us about finding limits? Science has often encountered limits where none were originally perceived to exist. Let us begin with the alchemists, who sought to transmute base metals such as lead into gold and silver. They could see no reason why such transformations should be impossible. Unbeknownst to the alchemists, there are physical barriers that prevent such transformations by the means at their disposal (ordinary chemical reactions). Such transmutation can be done utilizing particle accelerators, but the cost of doing so far exceeds the value of the precious metals obtained.

The steam engine, the first reciprocating man-made device harnessing the power of heat to do useful work, was invented at the end of the 17th century and first used in an industrial context by Thomas Newcomen in 1712.44 Early engines were used to pump water from mines. These early engines were extremely inefficient, yet the course of the eighteenth century saw steady and significant improvement in the ability of engines to convert heat into useful work. One can readily imagine an enthusiastic observer in the year 1800 waxing rhapsodic over the potential:

I can see no limit to the ability of steam engines to produce work. The rapid improvements over the past 100 years lead us inexorably to believe that from ever smaller amounts of coal, more work can be achieved, until all of our mines have their water pumped by the smallest of boilers.

Unfortunately for this hypothetical observer, there were two obstacles in the way, unknown in the year 1800: the First and Second Laws of Thermodynamics. The First Law limits the amount of energy which can be obtained by burning a quantity of coal, and the Second Law limits the amount of that energy which can be actually converted into useful work.

A similar situation existed with respect to the performance of telescopes of a given size, at one time thought to be unlimited. Early instruments were rather crude; better lens grinding methods and optical glass led to steady improvements. On this basis, it was thought that continued technological improvements would yield corresponding gains in optical performance and that this could continue indefinitely. Unfortunately, nothing was known at the time about the phenomenon of diffraction, which ultimately limits the performance of any device utilizing electromagnetic waves (or any other form of wave motion). As Harold Suiter has observed:

During the two hundred years between the invention of the telescope and the final acceptance of the wave theory of light, people actually believed that there was no limit on optical quality. If optics were made of exquisite quality, the central spot would shrink in size — or so opticians thought. They must have agonized when their optical masterpieces, on which they had worked so diligently, still showed that disk surrounded by a system of rings. We now know that there is a fundamental limit to imaging . . . For a given telescope focal length, the central spot (called the Airy disk) decreases linearly in diameter for larger apertures.45

The limiting factor in optical systems, of which the Airy disk is an effect, is diffraction, a function of the size of the optical elements and the wavelength of light.46 Other limitations discovered in course of scientific investigation are the universe's maximum speed limit, the speed of light, and the Uncertainty Principle, which limits the accuracy of simultaneous measurements of conjugate quantities such as position and momentum. All of these limits potentially have extra-scientific implications and people continue to draw such inferences from them. But contrary to dire predictions, not one of them has caused science to collapse in a heap.

On other hand, quantum mechanics made verifiable predictions, as did the laws of optics and thermodynamics. So Intelligent Design has to deliver on this, which implies that it must create a systematic, quantitative method of analyzing systems for complexity and determining accurately the likelihood that they could have arisen by chance. This will quantify the deep structure in the space of biological components and systems that Intelligent Design has effectively postulated. It will be a very difficult task, though not impossible, if the theory is correct. This will involve the need to indicate the basis for this structure in physical law and also to predict, in advance, what types of transformations will be impossible, given the appropriate boundary conditions and physical laws. The implications, as already noted, are enormous, not just with respect to evolution, but also medicine and many other areas of science and technology that rely on incremental system development.


Intelligent Design has been proposed as a new scientific hypothesis about the possibility of certain types of transitions occurring naturally. The thrust of its proposal is that biological (and other) entities may be partitioned into groups or classes within which transitions are possible, but between which they are not. This is an empirically testable hypothesis, one which contradicts a key assumption of Neo-Darwinism. As yet, only preliminary steps have been taken to formulate and test this hypothesis: Behe's irreducible complexity and Dembski's Design Filter. So no definitive statement about its truth can be made as yet. Of course, there is no guarantee that the Intelligent Design school will be able to reach its goals; nature may not be the way the theory postulates that it is. The notion of an "Intelligent Designer," for which the school is perhaps best known, is not part of its scientific basis, but rather an inference from it, or rather, an inference about reality based on the physical limitations in nature that the theory proposes. The school is, in some ways, its own worst enemy by not clearly distinguishing its scientific hypotheses from the extra-scientific inferences it draws. Intelligent Design does not deny naturalism — that is, it does not require science to begin utilizing non-natural forces and entities. It does dispute metaphysical naturalism, which asserts that all phenomena can be explained by science and that there is no other reality. But the latter assertion is a metaphysical inference from the former, which is itself an extra-scientific assumption. The common attacks on Intelligent Design, accusing it of being Creationism in disguise, a Trojan Horse, and of injecting theology into science, are completely baseless. Its critics should therefore concentrate on the scientific issues raised, ignore the metaphysics and eschew the propaganda arguments, however satisfying these may be. But Intelligent Design must also deliver on the science.


  1. On Pope Benedict, see Stacy Meichtry and Kevin Eckstrom, "Pope Echoes Arguments of Intelligent Design Advocates", Religion News Service, 9 November 1995, available at Cardinal Christoph Schönborn, "Finding Design in Nature," New York Times, 7 July 2005.
  2. For a more detailed discussion of the history of the evolution controversy, going back to the Ancient Greeks, see Fowler and Kuebler, The Evolution Controversy: A Survey of Competing Theories (Grand Rapids: Baker Academic, 2007), chapter 2.
  3. This is modern terminology; Darwin did not know about genes but surmised that there had to be some type of hereditary material subject to random changes.
  4. At least to some extent. Metaphysical naturalism can be held in various degrees; its most potent form couples it with reductionism, which claims that all phenomena can ultimately be explained by (reduced to) the laws of physics. This stronger form, especially, has clear theological implications; but it is widely held. Metaphysical naturalism contrasts with methodological naturalism, which only requires that science refrain from use of non-natural forces and elements, such as spiritual forces. The two are frequently confused.
  5. David Stove, Darwinian Fairytales (New York: Encounter Books, 1995).
  6. On the founding of the NCSE, see http: // The most significant scientific responses to Creationism include: Niles Eldridge, The Monkey Business (New York: Washington Square Press, 1982); Douglas Futuyma, Science on Trial (New York: Pantheon Books, 1983); and Tim Berra, Evolution and the Myth of Creationism (Stanford: Stanford University Press, 1990).
  7. Richard Dawkins, The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe Without Design (New York: W. W. Norton, 1996).
  8. Virtually all of it supported what we have termed historical evolution; a lesser but still significant amount supported common descent evolution. Little if any supported strong Darwinian evolution, and this was the crux of the problem.
  9. Phillip Johnson, Darwin on Trial (Downers Grove, IL: Intervarsity Press, 1993). First edition published in 1991 by Regnery.
  10. Michael Denton, Evolution: A Theory in Crisis (London: Burnett Books, 1985). Norman Macbeth, Darwin Retried, (Boston: Harvard Common Press, 1971). The Mathematical Challenges to the Neo-Darwinian Interpretation of Evolution, edited by P. S. Moorehead and M. M. Kaplan, (Philadelphia: Wistar Institute of Anatomy and Biology, 1967). H. P Yockey, Information Theory and Molecular Biology (Cambridge University Press, 1992). Fred Hoyle and N. C. Wickramasinghe, Why Neo-Darwinism Does Not Work (Cardiff: University College Cardiff Press, 1982).
  11. Jonathan Wells, The Politically Incorrect Guide to Darwinism and Intelligent Design, (Washington, DC: Regnery, 2006).
  12. Contrary to popular belief, Creationists do not reject the notion of common descent, as they need it to account for repopulation of the earth after Noah's flood. They do reject the idea that all of life could have come from a single ancestor, since they believe that only degenerative change can occur naturally.
  13. An intelligent agent might be able to reuse some pieces from the broken watch to create another device; but that would be intelligent design, of course.
  14. Michael Behe, "Darwin's Breakdown," in Signs of Intelligence, edited by William Dembski and James Kushiner, (Grand Rapids: Brazos Press, 2001), 93ff.
  15. They could, in theory, arise by "jumps" or "saltations", as they are called. But this idea, championed most recently by Richard Goldschmidt, has been resoundingly rejected by the Neo-Darwinian school in favor of small, incremental changes.
  16. The reader is cautioned about the multiple means of words such as "evolve" and "evolution"; they often change in the middle of an argument. Here "evolve" means just "change over time".
  17. Ibid., p. 5.
  18. Redrawn from Fowler and Kuebler.
  19. This point is frequently misunderstood by proponents of Neo-Darwinism, such as Dawkins.
  20. Yockey.
  21. Chromatin is the complex of DNA and protein which comprises chromosomes. Discussion of histone-4 at Flybase database,
  22. Fred Hoyle, Mathematics of Evolution (Memphis: Acorn Enterprises LLC, 1999), 103.
  23. As we have said, anything with a probability of 10-50 or lower cannot reasonably be expected to occur, and especially not occur multiple times.
  24. Mark Ridley, Evolution (Wiley-Blackwell, 2003) p. 345-346.
  25. Ridley, Mark, Evolution, p. 345-346.
  26. Ridley, Mark, Evolution, p. 345-346.
  27. By "finite probability" here, we mean that the transition has a reasonable chance of occurring in an appropriate time span. Given that the age of the universe is estimated at 1018 seconds, a probability on the order of 10-30, probably 10-15, is not reasonable.
  28. In reality, the space would not be two-dimensional but of much higher dimension. It is shown here in two dimensions for ease of understanding the principle involved.
  29. Some proponents of Neo-Darwinism, such as Miller, believe that there is no incompatibility with theistic belief. However, they do differ from their atheist colleagues in some key areas, such as their rejection of radical reductionism. This in effect means that there are certain things that cannot be done, though they are not related to the things which Intelligent Design claims cannot be done.
  30. Jonathan Wells, "Making Sense of Biology", in Signs of Intelligence, edited by William Dembski and James Kushiner (Grand Rapids: Brazos Press, 2001), 125.
  31. Wells, 133-134.
  32. Bruce L. Gordon, "Is Intelligent Design Science?," in Signs of Intelligence, 193-216.
  33. Not just any statement or theory about nature is scientific, of course. Philosophical and theological interpretations of nature are meaningful but not scientific. While the subject is beyond the scope of this article, science is concerned with explanations of natural phenomena by means of theories which utilize natural laws, testable empirically, and which have predictive power. In short, they allow us to manipulate nature, and therein lies their ability to be falsified by empirical evidence. Whenever science strays far from its empirical roots, it tends to become metaphysics, and ideologies substitute for the lack of empirical evidence. This seems to be occurring even in physics today, with its "theories of everything", and in particular, its untestable string theories. This has been a characteristic of the evolution controversy since 1859.
  34. Creationists theorize that high energy processes acting over short periods (instead of the usual low energy processes acting over long periods) formed the principal geological features of the earth. For further details on their theory, see chapter 6 of the book by Fowler and Kuebler cited in the endnotes.
  35. Eugenie Scott, Evolution vs. Creationism: an Introduction (Berkeley, University of California Press, 2005).
  36. Jeffrey Burton Russell, Inventing the Flat Earth: Columbus and the Modern Historians (New York: Prager, 1991), 1-5. Curiously, as noted in this book, the bizarre myth that people in Europe during the 15th centuries and earlier believed in a flat earth goes back to, of all people, Washington Irving. Somehow it became associated in school textbooks with Christopher Columbus, who supposedly used his belief in a round earth to justify his voyage to west to reach India; whereas others were afraid to do so for fear of falling off the earth. In fact, the problem all along centered on the distance and the uncertainty of such a long ocean voyage. The fact that this myth was taught for so long, and continues to be appear in textbooks, indicates how readily and uncritically many intellectuals accept anti-religious propaganda.
  37. See the appropriate chapters in Fowler and Kuebler, Evolution Controversy.
  38. Wells, 133ff.
  39. Quotation is from Carl von Weizsflcher, recalling his conversation with Nernst. Quoted in Wells, Politically Incorrect, 137.
  40. Xavier Zubiri, Naturaleza, Historia, Dios, sixth edition, (Madrid: Editora Nacional, 1974), 287. English translation by Thomas B. Fowler, Nature, History, God (Lanham: University Press of America, 1982).
  41. Ibid.
  42. In case the reader has any doubt about whether this is done, we may draw attention to remarks of German biologist Dieter Walossek, commenting on the fact that key fossils crucial to the Neo-Darwinian explanation of life were not found in Pre-Cambrian rock formations in a major Chinese discovery: "It doesn't matter if you find it [the missing fossil record] or not! . . . It's there! It's by law! All of the major taxa should have been there in the pre-Cambrian, whether proved or not!" Quoted in Fred Heeren, "Paleontologic Agitprop?," Insight (24 July 2000), 25.
  43. Dawkins, Richard, "The Necessity of Darwinism," New Scientist (15 April 1982), 130.
  44. D. S. L. Cardwell, From Watt to Clausius. The Rise of Thermo dynamics in the Early Industrial Age (Ithaca, NY Cornell University Press, 1971), 13-15.
  45. Harold Richard Suiter, Star Testing Astronomical Telescopes (Richmond: Willmann-Bell, 1994), 11.
  46. The Airy disk is a set of concentric rings observed around point sources of light in optical systems such as telescopes which have circular lenses or apertures.

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