Intelligent design

Intelligent design (ID) is the claim that fundamental features of the universe and living things are best explained by purposeful causation—a "higher intelligence." Proponents of ID argue that conventional biological theory of evolution through natural selection cannot explain the origin of many highly complex features of living organisms; they argue that some such features are better explained as the result of the intervention of a directed process that they describe as an intelligence. Critics of ID consider that these arguments are logically unsound and reflect a flawed understanding of modern biology; they believe that conventional biological theories explain the evolution of complex features of organisms elegantly and efficiently. Critics have also accused its supporters, many of whom believe in a personal God, of using ID primarily as a way to get around U.S. legal restrictions on teaching about God in the classroom. However, ID proponents claim that ID does not require belief in a personal God to support its arguments.

The U.S. National Academy of Sciences has concluded that, in its present form, ID cannot be regarded as scientific because the claims made by its proponents are not testable. See intelligent design movement for an account of political efforts to promote education about ID in schools.

An Argument for Intelligent Design

The classic design argument for the existence of an intelligent creator may be traced from ancient philosophy, through the works of medieval scholastics such as the philosopher-theologian Saint Thomas Aquinas (1225-1274), who argued that the existence of God could be deduced by reason.

In the 18th century, the theologian William Paley (1743-1805) introduced the "watchmaker analogy", one of the best known metaphors in the philosophy of science, as an argument for the existence of God. The argument is, in essence, as follows: Imagine walking on a pebbled beach, where the pebbles may be wonderfully shaped, beautiful in different ways, interesting and varied one from another. However interesting and beautiful you find them, you will not doubt that they are the products of purely natural causes. However, if amongst the pebbles you find a watch, even if you have never seen a watch before, you will immediately recognise it as qualitatively different from the pebbles. Inspecting it, from the intricacy of its design, and the clear purpose of that design, you will inevitably and correctly conclude that the watch is not a 'natural' object but an artifact, something designed by a powerful and intelligent agent.

Casual observation might similarly lead to the conclusion that, compared to a watch, even the simplest living form is incredibly complex, giving it the appearance of being designed for a purpose. There must be a designer, said Paley, "and that designer is God."

To many who believe in a Creator of the universe, the idea that the nature of living things might contain some evidence that they have been built purposefully rather than having been evolved by natural selection is a natural one. Some modern proponents of ID have accordingly broached this issue in a theoretical manner, by considering how, objectively, it might be possible to ascertain whether any particular object has been designed for a purpose as opposed to having evolved to fit an apparent purpose. Thus, William Dembski has argued that ID can be formulated as a scientific theory of information that has empirical consequences and which is devoid of any religious commitments.[1] By this view, intelligent design asks whether design can be detected in nature from purely scientific and mathematical considerations.

These proponents have proposed two criteria for deciding whether something has been designed in a way that cannot be accounted for by evolution through natural selection:

Irreducible Complexity

The first criterion relates to the notion of "irreducible complexity". Proponents of ID argue that irreducible complexity is a common feature of biological systems, and claim that it cannot plausibly be accounted for by evolution through 'undirected' natural selection. They argue instead that irreducibly complex systems must have arisen by some form of 'directed' evolution, i.e. towards a predetermined end.

A complex system is a system with many component parts, and which depends on many interactions between those component parts to perform its particular defined function. A system is irreducibly complex if removing of any one of these parts causes it to cease functioning. In No Free Lunch, William Dembski extends this basic definition, and states that a system is irreducibly complex

"if it includes a set of well-matched, mutually interacting, non-arbitrarily individuated parts such that each part in the set is indispensable to maintaining the system's basic, and therefore original, function."

Evolution by natural selection can proceed via successive minor alterations to the form of a structure, each of which results in an incremental improvement in fitness. One of the challenges for biologists is to explain how highly complex structures can have evolved in this way, as the apparent requirement that every intermediate form must have a selection advantage beyond the previous form severely limits the possible paths by which evolution might have proceeded (but it is possible for neutral mutations to accumulate). This constraint becomes particularly hard to reconcile with evolutionary explanations when the final form requires the involvement of each of several different elements for the final form to have any effective functionality. To proponents of ID, it is inconceivable that evolution by natural selection could have perfected multiple elements that only have any functionality when all are complete and assembled. To them, this is evidence of design in anticipation of a purpose, design according to a predetermined plan or 'blueprint'. This view however needs to be reconciled with mechanisms of evolution involving the many gene rearrangements that are possible from DNA recombination in which change over time is not incremental.

Specified complexity

The second criterion relates to the concept of "specified complexity". Complex structures can arise by natural processes, or by chance mechanisms and some may happen to have functionality that can be exploited; thus complexity per se is not evidence of intentional design, nor is the improbability of a structure or pattern arising by simple chance any evidence that it did not in fact arise by chance. However, proponents of ID argue that when a structure clearly fulfils a function that is essential for the function of living things - in other words when an outcome of an evolutionary process is absolutely required by the organism, then they argue that it is legitimate to consider how likely it is that that outcome could have been achieved through the chance processes that are said to be involved.

This is a complex argument, perhaps best explained by analogy. If an archer fires an arrow into the air at random, then it might land anywhere within a large area, so the probability of it landing at any particular place within this area is very low. In this case, we cannot draw any conclusions from the simple fact that the arrow lands at a place where it has a very low probability of landing - the arrow must, after all, land somewhere. However, if it lands in the very centre of a predrawn target that lies on the grass, then we can draw some conclusions - we might reasonably conclude that the arrow was not fired at random, but was aimed. This conclusion comes not solely from the fact that the outcome of firing the arrow was an unlikely complex event, but from this combined with the fact that it was a prespecified complex event - and the evidence of the specification is there in the form of the target on the grass.

Proponents of ID argue that there are features of living organisms that show such specified complexity. In other words, evolutionary processes have achieved outcomes that are complex in specific ways that, to proponents of ID, are suggestive of a predetermined purpose. These proponents argue that, for such outcomes, it is legitimate to calculate the probability of their occurrence according to the conditional probabilities of occurrence of the multiple chance elements that are involved. Dembski has argued that when such calculation produces an impossibly tiny figure for the conditional probability, then the hypothesis that it is the result of those chance processes must be discarded. He has proposed that conditional probabilities of the order of 10-120 are thus evidence of purpose, and thus of some 'directing intelligence'.

The statistical reasoning behind these arguments is complex, and the notion of specified complexity is not one that has been widely endorsed by professional mathematicians and information theorists. Part of the problem with applying this approach is the problem of assigning probabilities to particular molecular events in a reliable way. Another part of the problem arises from the difficulty in deciding what exactly constitutes specified complexity in a biological context.

Intelligent design and molecular biology

It is important to note that proponents of ID do not deny evolution, nor do they dispute the findings of molecular biology. On the contrary, they regard evolution as part of the mechanisms by which living things are designed, and they consider that molecular biology, by displaying the vast complexity that underlies even the simplest living things, provides evidence that makes explanations based on natural selection less plausible rather than more plausible. They note for instance that even the simplest single-celled organism requires several thousand genes for minimal functionality, each of which is encoded by a huge molecule containing many thousands of nucleotide bases organised in a very specific sequence. The DNA of even the simplest organism is a vast repository of precisely coded information.

Interestingly, watches have been found in living organisms. These are molecular timing devices that produce 'circadian rhythms', biological cycles with an innately determined time period. One such timekeeping mechanism has been found in blue-green algae, and it enables this microbe's genes to vary in their intensity of activity in rhythm with daily changes in sunlight. The biological watch inside the microbe is composed of widely distributed genetic components recruited from other signalling and information processing activities of the cell. In this case, the watch is constructed from simple molecular components that function regularly and predictably according to well characterised principles of biochemistry - the timing mechanism arises as a result of feedback loops whereby gene products regulate gene expression; in this case there seems no need to postulate ID in its construction.

Modern Evolutionary Synthesis

ID proponents argue that the natural selection mechanisms that have been used to explain the "apparent design" of numerous components and interactions of living organisms cannot explain all features of organism complexity, but their critics argue that they underestimate the sophistication of modern evolutionary theory, and in particular underestimate the creative capacity of natural selection.

The currently accepted account of the origin of living things is known as the "Modern Evolutionary Synthesis". This expresses the theory of evolution by natural selection in a form consistent with molecular biology and population genetics. By this theory, all extant living things are the products of descent with modification from common ancestors that lived 3-4 billion years ago. This theory (and an abundant array of evidence of evolutionary process that are seen in nature) explains all evolution as resulting from processes of natural selection among populations among which genetic novelty and novel combinations of genetic components are generated by many different genetic mechanisms. Natural selection is not the single simplistic process that the common characterisation "survival of the fittest" suggests, but embraces a very wide array of processes and pressures that lead to differential reproductive success in populations. The creative elements of natural selection emerge because, as selected genotypes accumulate over many, many generations and combine in different ways to produce novel phenotypes, this produces a huge diversity of organisms (today there are more than 10 million living species, a tiny percentage of all the species that ever existed).

There are important features that distinguish the 'design' achieved by natural selection from the kind produced by an intelligent designer. Natural selection is undirected in that it has no preordained plan, but is simply a result of the differential survival and reproduction of living beings. Natural selection and genetic mechanism provide powerful ways of generating novel features which are stored in the DNA code and passed on to the subsequent generations, but natural selection has no foresight, and so environmental changes may threaten the survival of organisms that were thriving. As a result, species extinction is common: it is estimated that, of all the species that ever lived on Earth, more than 99% are now extinct. Critics of ID see the evidence of massive extinction of species as inconsistent with the notion of intelligent design of the world we live in.

Although most descriptions of evolution assume that all mutations are random with respect to their effects on fitness, some biologists argue that there are important mechanisms which make favourable mutations more likely to occur. For example, Lynne Caparole has argued that natural selection favours some mechanisms that increase the likelihood of mutations in particular genes. These mechanisms produce considerable diversity in gene function amongst progeny, and this diversity can facilitate rapid evolution.

In particular, not all mutations rely on a random change to a single nucleotide base: some involve "mis-copying" the DNA, and these errors are very common for certain types of DNA sequence. For example, when a section of DNA contains a short repeated sequence of nucleotides, the number of repeats can be affected by a common "stuttering error" in DNA replication. The consequences will depend on exactly where in the genome the repeated sequence is located, but if, for example, it occurs in a region that affects how a bacterium responds to an antibiotic, then this diversity makes it more likely that some offspring will be antibiotic resistant. Thus, although accuracy is generally very important in reproducing a genome, some 'errors' are valuable, because the increased diversity of the offspring makes it more likely that some will be well fit to rapidly changing environmental conditions.

ID publications

Many books on ID have been written but few ID research papers or monographs have been published in conventional peer-reviewed scientific journals. However, ID advocates have created several journals devoted to publishing papers on ID (see Bibliography).

On 4 August 2004, a review article by Stephen Meyer, an ID proponent, appeared in the peer-reviewed Proceedings of the Biological Society of Washington, questioning conventional evolutionary explanations for the Cambrian Explosion and proposing ID as an alternative The review questions whether whether conventional biological theory can explain the information explosion evident during the Cambrian period. He says: "For neo-Darwinism, new functional genes either arise from non-coding sections in the genome or from preexisting genes. " He argues that the first leaves too much to luck and chance to be plausible, and the second has the problem that changes to existing genes are almost invariably deleterious. However this neglects many conventional accounts of how genes evolve, including probably the major known mechanism whereby novel genes arise, via gene duplication. Gene duplication is very common through evolution, and means that one copy of a gene can continue to sustain its normal function while the other is "surplus", and is free to accumulate mutations. Thus, by conventional explanations, a new gene doesn't have to be built from nothing - instead evolution is handed an already functional "toy" to play with.

Later, the Council of the Biological Society of Washington retracted the article. The managing editor for the journal at the time, the process structuralist Richard Sternberg, stated that the article had been properly peer reviewed by three well qualified referees. His decision to publish the paper nevertheless resulted in protests, and colleagues at the Smithsonian Institute, where he was employed, sought to discredit him and created what the The U.S. Office of Special Council (which is authorized to investigate allegations of prohibited personnel practices and activities prohibited by civil service law) called "a hostile working environment".

The lack of peer-reviewed articles is cited by some as evidence that ID is unscientific. ID supporters point to the Proceedings incident as showing the opposite: that the scientific climate is so prejudiced against ID that not even peer review is enough to overcome publication bias. ID proponent Dominque Tassot of the 'Center for Studies and Prospectives on Science', France, states the reasons as

"[Evolutionary scientists] live and think inside the paradigm of evolution. As Thomas Kuhn explained [in The Structure of Scientific Revolutions], theories are accepted or rejected in order to defend the dominant paradigm. Information which conflicts with that paradigm is set aside, it doesn’t get published."

Distinct from creationism

Most proponents of ID are also creationists, meaning they believe the universe was created by a deity or some other power beyond the full comprehension of mortal man. The hypothesis, however, is not the same as young earth creationism, which is a belief that the account of the creation of the universe and of life as given by the Bible is literally true. ID does not try to identify the designer as supernatural, nor does it try to establish the veracity of a particular narrative, although some leading proponents have stated that they believe the designer to be the Christian God. However, some proponents of ID see no conflict between it and evolution, and indeed see evolution as evidence of ID. Many proponents avoid any personification of the source of the ID, and so the focus of ID is different to that of arguments in natural theology, such as the teleological argument.

Criticisms of intelligent design

Proponents of ID argue that it is a scientific theory rather than a matter of religious belief, despite its base of support in the religious community. Courts in the USA have rejected this argument, in part because, for a theory to be 'scientific', it must generate predictions by which it can be tested; in other words, it must be open to attempted disproof.

Critics also hold that the postulate of an intelligent designer is not an explanation for life at all, but a "deus ex machina"--that is, an evasion of attempted explanation. They consider that ID is not a serious alternative to modern biological theory, in particular they consider that the complexity of biological systems can be well explained by conventional biological theory. They also consider that the tenet of ID that perfection in design needs a directing intelligence is inconsistent with many examples in nature of imperfections in design of organisms.

There are also many critics of ID who are religious, and who believe that the role of science is to seek natural, physical explanations of the world; although they believe that there is a God who created the world and life in it, for them this is a matter of faith not of science. Some critics doubt the intellectual honesty of ID theory, in that they consider that the only reason for promoting it is for the religious message that it implies, not for the intrinsic merits of the arguments.

While some proponents of ID see evidence of design in how well complex structures fit their purpose, molecular biologists see something very different in the fine details of those structures. So far from seeing genetic information being perfectly and economically fashioned to suit a given purpose, they see wastefulness, duplication, errors, and the detritus of now redundant genes in gene sequences — in other words, things they would expect to see as the residue of evolution by natural selection. In short, despite the appearance of efficient design at a high level, at a molecular level the design shows few signs of purposeful intelligence but many signs of chance processes. However, other proponents of ID note that ID does not imply perfection in design; after all, of the things that we know to be the result of intelligent design — things designed by people — few are perfect.

Conventional explanations for the origins of irreducible complexity

By the original common and uncontroversial definition, many biological systems are irreducibly complex. However, conventional evolutionary theory has no intrinsic difficulty in explaining how a system can have arisen by natural selection and yet appear to be irreducibly complex.

The key assumption which is problematic for ID is the assumption introduced by Dembski that the 'basic' function of a system is the same as its 'original' function. Biologists argue that, on the contrary, systems often evolved for one purpose and then were "exapted" to a different function. An example is the evolution of the wing. A small wing is useless for flying, so how could an aerodynamically effective wing ever have begun to evolve? However, small wings can be used to increase running speed. The aerodynamic consequences would have been incidental to the original function, but when species had evolved wings big enough for gliding, then this originally incidental benefit may have become the major focus of natural selection.

Another way by which a system might become irreducibly complex is via the eliminative actions of natural selection. For example, genes that were once important in precursor forms of an organism are likely to be eliminated by natural selection if they later become redundant. Many see this as analogous to the scaffolding and buttresses that are used to construct a building, which are then removed after the building is completed, thereby eliminating all evidence of exactly how the building was built. Thus, in a currently living organism we see only the final structure, with little remaining evidence of the "scaffolds" by which evolution built it.

Michael Behe further adapted Dembski's definition to apply it to evolutionary pathways:

"An irreducibly complex evolutionary pathway is one that contains one or more unselected steps (that is, one or more necessary-but-unselected mutations)."

This step makes an overt link between irreducible complexity and ID. As a definition, it can only be applied after assuming that evolution was directed (i.e. after assuming that there is any such thing as an unselected mutation)

Conventional explanations for rapid evolution of complex traits

ID proponents doubt that known genetic mechanisms can explain how organism evolve rapidly to achieve complex design outcomes, given that evolution requires rare mutations to arise through chance processes.

According to conventional biological theory, spontaneous mutations are the ultimate source of genetic novelty. One of the first estimates of the rate of spontaneous mutation in humans was made in 1930 by JBS Haldane , and was close to the current best estimates that every new human born harbours 100-200 novel gene mutations. Thus, in every generation, vast numbers of new mutations arise within a population; most are neutral, but a few are beneficial. However, for any beneficial mutation to become fixed in a population takes many generations. In 1957, Haldane estimated that, for a population of stable size, it takes about 300 generations for a new mutation to become fixed in a population. This conclusion was independent of the strength of selection pressure unless it is very intense, explaining the slowness of evolution by natural selection.

ID proponents have thus questioned, for example, whether there is enough time since the last common ancestor of humans and chimpanzees to explain the genetic differences between them.

Evolution does not proceed serially, by fixing first one genetic change and then the next, but in parallel: selection pressures apply simultaneously to all mutations that arise. How fast evolution by natural selection can occur when several traits are being selected for simultaneously is not clear, but Haldane warned that it could be very slow. However, he noted that evolution can occur much more rapidly when environmental pressures are changing. The last 10 million years has seen dramatic global changes in climate, and any species, over such a timescale, will encounter many other changes in selection pressure through the ever-changing ecological balance.

Modern genetics has identified many mechanisms that enable some organisms to evolve rapidly in a quasi-directed fashion In multicellular organisms, sexual selection is one of these, and Mobile DNAs are another (See DNA and molecular evolution.) The human immune system is a good illustration of such mechanisms. The creation of highly adapted antibodies that neutralise poisons and pathogens to which the body has never been exposed before, occurs by such natural selection events in rapidly evolving cells of the immune system (see Clonal Selection Theory of Acquired Immunity). Conversely,pathogens use equally powerful genetic strategies (such as sets of alternative genes for related structures) to evolve rapidly and to deftly switch their antigenic features so as to evade the immune system.

Could the flagellum have evolved by natural selection?

Many ID arguments are based on the notion that every piece of a biological machine must be assembled in its final form before anything useful can emerge.

For example, the flagellum is a complex structure that allows bacteria to move; up to 40 different proteins must be arranged in a very particular way for the flagellum to work. Proponents of ID argue that a flagellum is useless if it won't move, that it needs all of its parts to function, and that it is hard to conceive that it could have arisen by classical natural selection. Natural selection involves gradual cumulative mutations, each of which produces a gain in fitness; the ID argument is that the flagellum has adaptive value only when fully assembled - the intermediates are useless.

To biologists generally, this idea is a misconception. Evolution by natural selection produces complex biochemical machines by copying, modifying, and combining proteins that were previously used for other functions. Evolutionary theorists argue that many features of organisms evolved to fit one function and were then adapted through natural selection to fulfill a different function. By this view, natural selection is not a single path, but a multiply branching path with many dead ends, with many branching points where genes were duplicated, and with many changes of direction where the 'destination' changed.

Flagella are modular devices and the modules serve other functions

Flagella are modular devices, that exist in thousands, if not millions of versions in nature. Many radically different variations of flagella design occur, some rotate in two directions, others only in one. However, there is evidence that the flagellum structure arose from structurally simpler structures by gene duplication and further evolution. It appears that the flagellum proteins evolved from just two precursors (a proto-flagellin and a proto-rod/hook protein) by several rounds of gene duplication and diversification.

Many of the versions of flagella lack many of the particular components that, in other versions, seem to be irreplaceable. For example, in Gram positive bacteria the P- and L-rings present in the flagella of Gram negative bacteria are missing.

Relatives of some flagellum proteins are used in other bacterial structures. A protein similar to flagellum component FlgA is used in assembly of Type IV pili surface appendages; these are superficially like flagella, but have no major role in movement. Flagellum protein FlgJ contains a region that digests the rigid bacterial cell wall during flagellum insertion through the wall, and many similar bacterial proteins do other tasks.

All flagella include modular subcomponents called T3SS that are widely used by bacteria for secretion of proteins. This T3SS submodule is used to assemble the flagella, but is also used to secrete other molecules from the cell. There are also versions of flagella that are defective in providing motility, but which have a functional role in infection (e.g. in the pathogen Brucella melitensis)- evidence that components of flagella have useful biological functions apart from movement.

The modularity of flagella is not unusual in complex biological structures. Bacteria possess a wide variety of complex structures such as pili, secretion systems, and conjugation machinery. As a (richly documented) general rule, any particular module may have several different roles (in for example, secretion, uptake, protein of DNA injection or twitching motility) and complex structures are built by "mixing and matching" different sub-components. This general rule provides evolutionary explanations of biodiversity.

In summary, many components of the flagellum are used by bacteria for other functions, such as for injecting poisons into other cells, and these components are not irreducibly complex in these other functions. Accordingly, biologists think that different elements of the flagellum evolved separately to fulfill other functions in ancestral organisms. They thus think that motility arose relatively late in bacterial evolution, after many of the elements used in the flagellum had already evolved.

Does Science Stand in Opposition to Intelligent Design?

The modern scientific ethos involves two principles between which there is continual tension. One principle is that of skepticism — in the sense of doubt about all that we believe, not in the sense of denial of alternative views. However, a second principle is conservative: scientists build theories on conventionally accepted foundations, without which there is no common language in science and no progress. Thus scientists resist attacks on the foundations from which they proceed, and a theory that has led to significant increases in knowledge and understanding and which continues to direct science in productive ways will be retained unless an alternative theory promises a significant enhancement of understanding.

No scientist should consider it illegitimate to question current evolutionary theory: on the contrary, the right to challenge ‘’anything’’ that we accept is an essential element in science, and science that denies the right to challenge becomes mere dogma. The core question addressed by some ID proponents - is there a way in which we can test, by analytical processes, whether or not a feature has evolved by the known mechanisms of natural selection?, when phrased in this way, is consistent with scientific approaches, and indeed, is a question often asked by biologists. So why is there such antagonism between scientists and the proponents of ID?

First, for scientists who seek to test a theory, any test should be designed without preconception about what the “right” answer is. The Intelligent Design Movement is accused by some as having an ‘’agenda’’ – it presupposes that there is an intelligent creator, and seeks to provide an apparently scientific buttress for this preconception. This approach, to critics, is anti-scientific.

Second, the ID movement seeks to draw conclusions far beyond the bounds of testable knowledge. Thus, it is reasonable to question whether a proposed explanation for, say, the evolution of flagella, is logically tenable given present evolutionary theory. However, if the explanation is incomplete, scientists consider it unreasonable to conclude that the whole edifice of evolutionary theory is, therefore, wrong and must be replaced by a theory that invokes an extravagant and untestable presumption. In their own challenges to accepted theory, scientists respond to anomalies by considering how they might be reconciled by minor adjustments to existing theory. Thus they first adopt a conservative approach, and do so because of the wider context – their awareness of how much modern theory has contributed to knowledge, and of what is at stake, for the progress of science, in rejecting it. The process of falsification of hypotheses is important in science, but a major theory is not something to be rejected lightly.

All scientists are creatures of the times in which they live, subject to the same pressures of society, politics and economics as others, and individually they have prejudices and preconceptions which can never be wholly shed. However, some scientists throughout history have resisted attempts by religious authorities, society or governments to impose interpretations that are not warranted objectively. This resistance has not been universal, nor has it been consistently effective; nevertheless, such resistance is celebrated by scientists as a mark of integrity. Today, an ethical scientist might accept funding from a pharmaceutical company to study the actions of a drug, but will reject funding that is conditional on a given outcome: a study should be designed in such a way as not to favour a particular preconceived conclusion. By this view, the ID movement, insofar as it seeks to provide evidence to support a particular thesis, is anti-scientific, and scientists who accept funding that it is conditional on a given conclusion are in breach of scientific ethics.

Finally, science is about things that can be tested, whereas religious faith is a matter of inner conviction, and is not externally testable. The existence of a God is not something that is, even in principle, capable of disproof, so any explanations that invoke this are, by this view, unscientific. Some proponents of ID have tried to separate what are legitimate challenges to current theory from their religious conclusions. Thus far, few scientists are convinced that this separation is a sincere attempt at objectivity, few believe that ID poses a significant challenge to current understanding, and many believe that its proponents are so compromised by their preconceptions, by the nature of their funding, and by the manner in which they publish their work, as to place them outside the bounds of normal science.

This is not to say that the proponents of ID are necessarily wrong in their analyses of weaknesses in conventional understanding, or in their conclusions. For scientists, all knowledge and understanding is provisional. Our scientific understanding is the best we have now, but if science continues to progress, our present understanding will be, in time, superceded by something different and better.