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The NAS elaborates on this definition as follows:
In science, explanations must be based on naturally occurring phenomena. Natural causes are, in principle, reproducible and therefore can be checked independently by others. If explanations are based on purported forces that are outside of nature, scientists have no way of either confirming or disproving those explanations. Any scientific explanation has to be testable -- there must be possible observational consequences that could support the idea but also ones that could refute it. Unless a proposed explanation is framed in a way that some observational evidence could potentially count against it, that explanation cannot be subjected to scientific testing.
Numerous others have expressed similar views. For example, Michael Ruse, a philosopher of science who testified in the 1981 Alabama creationism case, describes science as a discipline that (1) is guided by natural law, (2) is explanatory by reference to natural law, (3) is testable against the empirical world, (4) reaches conclusions that are tentative, and (5) is falsifiable [Pennock1999, pg. 5]. Similarly, philosopher Robert Pennock emphasizes that science is based on "methodological naturalism," which requires scientists to seek explanations based upon what can be observed, tested, replicated, and verified, and that this is a necessary "ground rule" of science today, not an optional afterthought [Jones2005, pg. 65]. For additional discussion of methodological naturalism, see Methodological naturalism.
I shall certainly admit a system as empirical or scientific only if it is capable of being tested by experience. These considerations suggest that not the verifiability but the falsifiability of a system is to be taken as the criterion of demarcation. ... It must be possible for an empirical scientific system to be refuted by experience.
Popper's ideas remain highly influential in scientific research even to the present day. For example, several prominent scientists have recently expressed concern about whether it is prudent to continue pursuing string theory, given that practitioners have not yet been able to derive empirically testable consequences even after 25 years of effort. Physicist Lee Smolin, for example, writes, "A scientific theory that makes no predictions and therefore is not subject to experiment can never fail, but such a theory can never succeed either, as long as science stands for knowledge gained from rational argument borne out by evidence." [Smolin2006, pg. 352].
However, Popper's ideas do have some limitations, some of which were pointed out by Popper himself in his earliest works. To begin with, in most real modern-day scientific research, major theories are seldom falsified by a single experimental result. There are always questions regarding the underlying experimental design, measurement procedures, and data analysis techniques, not to mention statistical uncertainties. Often multiple follow-on studies, in some cases extending over many years, are necessary to conclusively decide the hypothesis one way or the other. For example, 13 years elapsed between 1998, when two teams of researchers discovered that the expansion of the universe is accelerating, and 2011, when the lead scientists of the two teams (Saul Perlmutter of the Lawrence Berkeley National Laboratory and Brian Schmidt of the Australian National University) were awarded the Nobel Prize in physics, allowing time for these startling results to be very carefully scrutinized in reams of follow-on studies by researchers worldwide. In this sense, scientists are more like detectives, in that they must follow leads and hunches, examine evidence, and tentatively proceed with the most likely scenario. Seldom, if ever, are scientific results black-and-white from day one.
Some additional discussion of Popper's philosophy and the notion of falsifiability are given in Postmodern.
The scientific peer review system operates as follows. When a scientist or team of scientists completes a research project, they document their methodology and results in a manuscript, which is then submitted to a journal or refereed conference for peer review. The editor (if sent to a journal) or technical papers chair (if sent to a conference) then privately distributes the manuscript, electronically in most cases, to at least three other persons, chosen due to their knowledge and expertise in the manuscript's topic. These persons, known as "referees," rate the paper on criteria such as relevance to the journal or conference's charter, clarity of exposition, acknowledgement of prior related work in the field, methodology, data collection and analysis, whether conclusions are justified based on the data and analysis presented, and the overall importance of the authors' contribution. When the editor (or chair) receives these reports, he/she decides whether: (a) to accept the manuscript as-is, (b) to accept the manuscript, provided that some relatively minor items identified by the referees are corrected, (c) not to accept the paper as-is, but to reconsider if some relatively significant items are corrected or improved, or (d) to reject the manuscript, and to not encourage resubmission.
For many scientific journals and conferences, fewer than 25% of submissions are initially accepted or accepted pending minor revision. The others are rejected, require major revision (sometimes more than once), are subsequently submitted to another, more relevant journal or conference, or are never formally published. The process of documenting scientific results in papers and the process of peer review both require a huge amount of time and effort on the part of scientific researchers, but the resulting peer-reviewed journals and conference proceedings are of significantly higher quality as a result. For additional details and discussion, see Peer review.
Similarly, the issue of whether intelligent design qualifies as science came up in the 2005 Dover, Pennsylvania case. Catholic theologian/philosopher John Haught testified that the arguments advanced by intelligent design writers are not new scientific arguments, but instead harken back to at least the 13th century, when Thomas Aquinas argued that since nature appears to have a complex design, there must have been a Designer. Essentially the same argument was again advanced in the 19th century by William Paley. And although modern intelligent design authors do not publicly identify this designer with God, Haught points out that anyone familiar with Western religious thought would immediately make the connection with the Judeo-Christian God [Jones2005, pg. 25].
In the same case, scientific philosopher Robert Pennock testified that because intelligent design teaches that the features of the natural world were produced by a transcendent, immaterial, non-natural being, it is thus a religious, not a scientific movement [Jones2005, pg. 28]. It is telling that intelligent design writer Michael Behe, in his court testimony, acknowledged that the "plausibility of the argument for [intelligent design] depends upon the extent to which one believes in the existence of God." But as Judge Jones noted in his opinion, there is no evidence "that any other scientific proposition's validity rests on belief in God" [Jones2005, pg. 28].
One other key item of evidence noted in the Dover trial was the testimony of Barbara Forrest, who found, through her careful analysis of the intelligent design textbook Of Pandas and People, that this book was merely a lightly edited reworking of an earlier creationist text, where, for example, the words "creation," "God," "creationism," and "Genesis" had been systematically replaced by less overtly religious words such as "intelligent design" and "designer." Other than superficial changes such as this, many of the same arguments and examples were retained, almost word for word [Jones2005, pg. 32-35].
In his decision, Judge Jones further noted that the intelligent design movement had failed to gain acceptance in the scientific community. In particular, it had not generated any peer-reviewed publications, as is required of all legitimate scientific research. He ultimately concluded that intelligent design fails to meet the essential ground rules to qualify as science [Jones2005, pg. 70].
For additional background on creationism and intelligent design, see Creationism and Intelligent design. For additional details on the 1981 and 2005 court cases, see Court cases.
This lack of peer-reviewed publications, or even serious attempts at submitting material for peer review, presents a severe obstacle to creationism and intelligent design being taken seriously in the scientific world. If creationist and intelligent design writers (individually or collectively) believe that any of their technical issues have significant merit on purely scientific grounds, why do they not compose them into well-researched and well-analyzed articles and submit these articles to recognized peer-reviewed scientific journals?
After all, as emphasized in a recent Science letter signed by numerous prominent scientists (after brief mention of the prevailing theories of geology, big bang cosmology and evolution), "Even as these are overwhelmingly accepted by the scientific community, fame still awaits anyone who could show these theories to be wrong." [Gleick2010]. The only reasonable inference from the lack of publications is that these writers themselves recognize that the arguments and data that they have presented to date would not meet the rigorous standards required of serious peer-reviewed scientific literature.
But by its very definition, science cannot render judgments one way or the other on the existence or nature of God, or on processes of creation that are hypothesized to lie outside the realm of natural law. What's more, science, properly defined, can say nothing about the ultimate purpose of creation, nor can it provide any fundamental direction for ethics or morality. These questions are better pursued through the arena of religion.
By the same token, creationism and intelligent design also lie outside the realm of science. While these writings are often cast in scientific terminology, and may be persuasive to those who lack professional training in the particular fields of science in question, these writings do not meet the standards of serious, peer-reviewed scientific research.
See also Peer review and Science-answers.