The earliest foundations of the scientific method, as distinguished from earlier methods of acquiring knowledge such as [Aristotelian logic]?, [divine inspiration]? or the [Socratic method]?, are often credited to Roger Bacon and Galileo Galilei. Later contributions by Francis Bacon, Rene Descartes, Karl Popper, and others added to the understanding of the methods of science. (See History of Science and Technology.)
The notion of a scientific method has been criticized for providing an overly simplistic and perhaps misleading view of what scientists do and how they view the world. An alternative model of the process of science has been proposed by Thomas Kuhn, who suggested that sociological mechanisms were important in science. Some "post-modernists" have gone even further and claimed that sociological mechanisms are the only content of science and thus, implicitly, science differs only in the details of its sociology from, say Druidic tree spirit beliefs, as a description of reality (no offense is intended to Druids, if there be any remaining).
Other viewpoints hold that the scientific method is not so much a way to acquire knowledge as it is simply a procedure for validating knowledge already gathered. Regardless, there is a general consensus that the scientific method provides at least a mechanism to improve knowledge and eliminate errors and cultural biases, unlike more dogmatic methods of reasoning.
The scientific method is often described today as comprising these main actions:
These steps are repeated continually, building a larger and larger set of well-tested hypotheses to explain more and more phenomena. They are generally performed in the orderly manner above, but not necessarily: for example, theoretical physicists often invent totally new hypotheses before using them to decide what phenomena to observe.
Scientific observation consists mostly of making careful measurements (See Measurement). It is important that the methods of gathering the evidence be disclosed, particularly when the evidence being presented has not been previously reported (as with the results of previous experimentation). This makes it possible for others to repeat the observations independently to check for bias. Failure to disclose methods and techniques have caused several famous scandals, for instance P Kammerer's discredited work with toads.
Scientists also try to use operational definitions of their measurements. That is, measurements and other criteria for observation are defined in terms of physical experiments that can be performed by anyone, rather than being defined in terms of abstract ideas or common understanding. For example, the term "day" is useful in ordinary life and we don't have to define it precisely to make use of it. But in studying the motion of the Earth, you have to be more careful, so science makes two operational definitions: a solar day is the time between observing the sun at a particular position in the sky and observing it in the same position the next time; a sidereal day is the time between observing a specific star in the night sky at a specific position, and that same observation made the next time. These are useful since they are slightly different as a result of how the Earth moves, and properly using one or the other avoids problems. In particular, you will come to notice that the length of the solar day varies over the course of a year; you can then make a new operational definition of mean solar day as the average of these and study further.
In the hypothetical stage, scientists use their own creativity, or any other methods available, to invent possible explanations for the phenomenon under study. The most important aspect of an explanation is that it must be falsifiable. The scientist should also be -- but need not be -- impartial, considering all known evidence, and not merely the evidence which supports the hypothesis being developed. This makes it more likely that the hypotheses formed will be relevant and useful. Explanations should also satisfy the principle of Occam's Razor; i.e., the hypothesis is expected to contain the least possible number of unproven assumptions. For example, after a storm a tree is noticed to have fallen. Based on this evidence of "a storm" and "a fallen tree" a reasonable hypothesis would be "a lightning bolt has hit the tree"--a hypothesis which requires only one assumption--that it was, in fact, a lightning bolt (as opposed to a strong wind or an elephant) which knocked over the tree. The hypothesis that "the tree was knocked over by marauding 200 meter tall space aliens" requires several additional assumptions (concerning the very existence of aliens, their ability to travel interstellar distances and the alien biology that allows them to be 200 meters tall in terrestrial gravity) and is therefore inferior. Certainly more than one hypothesis can be entertained, and some of them might even be complex and require too many assumptions, but Occam's Razor is only a rule of thumb for quickly evaluating which hypotheses are likely to be fruitful; it is not a strict rule, nor an inevitable aspect of the scientific method.
It was once thought that science was based on [inductive reasoning]?; that is, if one observes the same thing many times without ever observing an exception, one can conclude from that observation alone that the phenomenon is consistent. This was the view of Francis Bacon and other empiricists, for example. Douglas Hume's critique of induction settled its use in validation or proof. In the modern understanding of scientific method, induction serves only as a means of suggesting hypotheses; these still must be tested by experiment and evaluated in the same way as other hypotheses.
Hypotheses are also considered superior to other possible ones if they have more predictive power; that is, if there are many possible observations one might make that would falsify the hypothesis. The hypothesis that "all physical matter turns into chocolate when no one is looking" cannot be refuted, for by definition it cannot be tested, and is therefore not a proper scientific hypothesis. A hypothesis that predicts that "light bends in a strong gravitational field" (one aspect of Einstein's theory of general relativity) is a strong hypothesis as it suggests concrete measurements which can be conducted to support or refute the claim. Using the prior "fallen tree" example, the hypothesis predicts that the fallen tree will exhibit scorch marks or similar markings consistent with a lightning strike, and that meteorological records of the storm are likely to show that lightning occurred.
Note that [deductive reasoning]? is generally used to predict the results of the hypothesis. That is, in order to predict what measurements one might find if you conduct an experiment, treat the hypothesis as a premise, and reason deductively from that to some non-obvious conclusion, then test that conclusion. For example, Einstein's equations implied that time operated differently than had been thought, but in a way that would only appear in conditions that humans had never seen. Assuming his equations were accurate and reasoning deductively from them, they implied that a clock sent on a fast spaceship would slow down compared to an identical clock left on Earth, while if he was wrong, the clocks should stay synchronized. This prediction has since been tested, and a moving clock does indeed slow down with respect to its stationary twin.
Probably the most important and universal aspect of scientific reasoning is verification: every hypothesis must be tested by performing appropriate physical experiments and measuring the results. Ideally, the experiments performed should be fully described so that anyone can reproduce them, and many scientists should independently verify every theory with multiple experiments.
Scientists should also attempt to design their experiments carefully. For example, if the measurements to be taken are difficult or subject to observer bias, one must be careful to avoid distorting the results by the experimenter's wishes. When experimenting on complex systems, one must be careful to isolate the effect being tested from other possible causes of the intended result (this is called a controlled experiment). In testing a drug, for example, it is important to carefully test that the supposed effect of the drug is produced only by the drug itself, and not the placebo effect or by random chance. Doctors do this with what is called a double-blind study: two groups of patients are compared, one of which receives the drug and one of which receives a placebo. No patient in either group knows whether or not they are getting the real drug; even the doctors who interact with the patients don't know which group they are giving the real drugs to and which they are giving fake drugs, so their knowledge can't influence the patients either.
Any hypothesis, no matter how respected or time-honored, must be discarded once it is contradicted by new evidence. Hence all scientific knowledge is always in a state of flux, for at any time new evidence could be presented that contradicts long-held hypothesises. A classic example is the Wave Theory of Light--although it had been held to be incontrovertible for many decades, it was refuted by the discovery of the photoelectric effect. The currently held theory of light holds that photons (the particles of light) also behave as waves under some circumstances. In the tree example, the lack of scorch marks or of reports of lightning, combined with reports of hurricane force winds would cause the original hypothesis to be re-evaluated as less probable and a new one ("The tree was knocked over by strong winds") to be proposed. Choosing between the two would require additional tests. Note, however, that the tree example involves "historical tests" and illustrates one of the differences between an experimental science (e.g., physics) and an observational one (e.g., paleontology or stellar evolution).
Further, the experiments that reject an hypothesis should be performed by as many different scientists as possible to guard against bias, misunderstanding, and fraud. Scientific journals use a process of peer review, in which scientists submit their results to a panel of fellow secientists (who may or may not know the identity of the writer) for evaluation. Scientists are rightly suspicious of results that do not go through this process; for example, the cold fusion experiments of Fleichman and Pons were never peer reviewed--they were announced directly to the press, before any other scientists had tried to reproduce the results or evaluate their efforts. They have not been reproduced elsewhere.
One school of thought asserts that the scientific method (and science in general) relies upon basic axioms or "self-evident truths" such as realism and consistency. While it is true that many scientists believe these things and do assume them in their everyday work, the method itself does not rely on them: all such assumptions are just part of the hypotheses being tested, and many are subject to tests as well. For example, one of the "common sense" ideas that scientists believed for a long time is that any measurable property of an object is something that exists in the object before it is measured, and our measurements are merely observations of that pre-existing condition; Quantum mechanics rejects this idea, because experiments have contradicted it.
Some believe that scientific principles have been "solidly" established, beyond question. Some scientists themselves may indeed feel that way, having come to rely upon many of the results of science without having done all the experiments themselves; after all, one cannot expect every individual scientist to repeat hundreds of years' worth of experiments. Many scientists even encourage an attitude of skepticism toward claims that contradict the current state of common knowledge; but that only means such claims must meet a higher burden before being accepted, not that they can never be accepted. In the extreme some, including some scientists, believe in scientific principles, or even "science" itself, as a matter of faith in a manner similar to those of religious believers. However, neither science nor scientific method itself rely on faith; all scientific facts (i.e., measurements) and explanations (i.e., hypothesises) are subject to test, and will eventually be rejected as the best available hypothesis upon new evidence falsifying them.
The terms "scientific theory" and "scientific laws" are used with exceptional frequency by the general populace, and in light of the discussion above, the actual meanings of these terms merit an explanation.
In general a hypothesis is a contention that has not (yet) been sustained or refuted, as one or more predictions made from it have not yet been tested. However, once the predictive phase has been carried out (at least to some degree) and there is some experimental evidence that supports the hypothesis then it may begin to be referred to as a "theory". Some scientists further reserve the term "theory" for hypotheses that are very well-tested and that explain a large number of phenomena, such as the theory of evolution by natural selection.
A theory that has withstood the test of time (and many experimental tests), and that has not been falsified by credible, repeatable experimental evidence or observation may eventually acquire the status of a "law". However "scientific law" may be gnenerally thought of as an inappropriate and misleading term, for it implies a permanance and immutability of "fact" which science and the scientific method simply does not produce. It is a fundamental tenet of the scientific method that all "results" are provisional, and this must include the so-called "laws". Newton's "law of gravitation" is a famous example of a "law" that (albeit only in some circumstances unknown for some time afterwards) was found to be in error (see general relativity for a discussion of this topic) as a general description of the behavior of matter in motion and of the behavior of gravity.
It is unlikely that anyone would dispute that the application of the "scientific method" is a standard approach for (retroactively) testing the status of any scientific hypothesis or theory. Certainly no respectable peer-reviewed journal would publish any scientific work if it could not be demonstrated that its hypotheses can (in principle) be shown to be in accordance with the method as presented here. What is regularly disputed is the contention that scientific research is, in fact, consistently carried out in the procedural manner described above.
The scientific method, as presented, offers no guidelines for the production of new hypotheses. Scientific folklore is strewn with stories of scientists describing a "flash of inspiration" which then motivated them to look for evidence to support their assertion. Some accounts tell of scientists operating on a "hunch" or a "gut instinct" prior to obtaining any supporting evidences for their hypotheses. Likewise, many scientists will follow a theory because it is "elegant" or "beautiful"; or choose not to follow a theory because it is "counter-intuitive". These psychological reactions are quite common in scientists as with us all. But regardless of its source, it is fundamental that science (through its practicioners, scientists) test all hypotheses (or theories if you prefer); the results of those tests are the only criteria for retaining a hypothesis. Neither beauty nor intuitive conviction nor prestige of proposer is acceptable as a substitute.
Another criticism of the scientific method (as here presented) is that it fails to acknowledge the incalculable impact that mathematics has had on scientific research and direction. A hypothesis about the physical world that is based solely on implications derived from mathematical analysis can hardly be said to be in accordance with the "observational" phase of the scientific method (a purely mathematical property cannot properly be called a "fact" about the physical universe). Nonetheless scientific history can recount hundreds of such occasions where a theory has been proposed based solely on mathematical findings, notably some aspects of quantum mechanics and the application of [fractal geometry]? to certain areas within biology, among others. A simple reply to this is that it does not affect the value of the scientific method itself to add an extra prior step: that is, to use mathematical results to choose what to observe and to inspire hypotheses. The unreasonable application of purely theoretical structures to actual physical reality (for example, mathematical group theory with regard to particle energy transitions) is a long-standing concern and is as yet unexplained.
A final criticism of the scientific method is that it provides no guidelines for choosing between two equally possible hypotheses (that meet all the other requirements for simplicity, evidential compliance, etc). Any scientist in such a situation will tend to support the hypothesis which "feels the best", and hence is likely to make a subjective selection influenced by cultural bias. Of course, if there is no physical experiment to distinguish one scientific hypothesis from another, then it cannot matter in one's ordinary life which one chooses to support. It is not the goal of science to answer all questions, nor even to explain phenomena which are not experimentally accessible. Science does not produce truth, it merely improves the currently best hypothesis about some aspect of reality. It cannot therefore, be a better source of value judgements or of answers to concerns in public policy. What one projects from the currently most reasonable scientific hypothesis into other realms of interest is not a strictly scientific question and the scientific method offers not assistance for doing so.
See also [Philosophy of science]?, [Bayesian logic]?, epistemology, Faith.
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