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Scientific method
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Scientific method

The scientific method is a sequence or collection of processes that are considered characteristic of scientific investigation and the acquisition of new scientific knowledge based upon physical evidence.

Table of contents
1 History
2 The scientific method
3 Philosophical issues
4 Scientific method and the practice of science
5 Annotated list of related issues
6 External links


The development of scientific method is indivisible from the development of science itself.

In his enunciation of a 'method' in the thirteenth century, Roger Bacon was inspired by the writings of Arab alchemists, who had preserved and built upon Aristotle's portrait of induction. Bacon described a repeating cycle of observation, hypothesis, experimentation, and the need for independent verification. In the 17th century Francis Bacon attempted to describe a rational procedure for establishing causation between phenomena.

Galileo Galilei introduced quantitative experimentation and mathematical analysis, which permitted the enunciation of general physical laws. Isaac Newton systematised these laws, becoming a model which other sciences sought to emulate.

Attempts to systematise the scientific method were faced with the Problem of induction, which points out that inductive reasoning is not logically valid. David Hume set the difficulty out in detail. Karl Popper, following others, argued that a hypothesis must be falsifiable; that is, it must be capable of disproof. Difficulties with this have led to the rejection of the very idea that there is a single method that is universally applicable to all the sciences, and that serves to distinguish science from non-science.

The question of how science operates has importance well beyond scientific circles or the academic community. In the judicial system and in public policy controversies, for example, a study's deviation from accepted scientific practice is grounds for rejecting it as "junk science" or pseudoscience.

The scientific method

The essential elements of the scientific method are iterations and recursions of the following four steps:

  1. Characterization
  2. Hypothesis (a theoretical, hypothetical explanation)
  3. Prediction (logical deduction from the hypothesis)
  4. Experiment (test of all of the above)

This can be called the hypothetico-deductive method. These activities do not describe all that scientists do (see below). The 4-step method described above is often used in education. Teachers using inquiry as a teaching method sometimes teach a slightly modified version of the scientific method in which "Question" is substituted for Observation.

Science is a social activity. The process is subject to evaluation by the scientists directly involved, or by the scientific community, at any or every stage. A scientist's theory or proposal is accepted only after it has become known to others (usually via publication, ideally peer reviewed publication) and criticised. See the list of unsolved problems in science, for example.


The scientific method depends upon the careful characterization of the subject of the investigation.

Observation demands careful measurement and the use of Operational definitions of relevant concepts. When the terms used are formally defining, they acquire exact meanings which do not necessarily correspond with their use in natural language: for example, mass and weight are quite distinct concepts, but the distinction is often ignored in everyday life.

New theories may arise when it is realised that words used have not previously been clearly defined. For example, Albert Einstein's first paper on relativity begins by defining simultaneity and the means for determining length (which were skipped over by Isaac Newton with "I do not define time, space, place and motion, as being well known to all") and proceeds to demonstrate that, given these definitions, certain widely accepted ideas (absolute time; length independent of motion) were invalid.

Hypothesis Development

A hypothesis includes a suggested explanation of the subject. It will generally provide a causal explanation or propose some correlation.

Observations have the general form of existential statements, stating that some particular instance of the phenomena has some characteristic. Causal explanations have the general form of universal statements, stating that every instance of the phenomena has a particular characteristic. It is not deductively valid to infer a universal statement from any series of particular observations. This is the problem of induction. Many solutions to this problem have been suggested, including falsifiability and Bayesian inference. Bayesian inference has been claimed as a suitable logical basis for discriminating between conflicting hypotheses. This method uses an estimate of the degree of belief in a hypothesis before the advent of some evidence to give a numerical value to the degree of belief in the hypothesis after the advent of the evidence. Because it relies on subjective degrees of belief, it is not able to provide a completely objective account of induction.

Scientists use whatever they can—their own creativity, ideas from other fields, induction, or even systematic guessing, or any other methods available—to come up with possible explanations for the phenomenon under study. There are no definitive guidelines for the production of new hypotheses. The history of science is filled with stories of scientists claiming a "flash of inspiration", or a hunch, which then motivated them to look for evidence to support or refute their idea. Michael Polanyi made such creativity the centrepiece of his discussion of methodology.

Prediction from the hypothesis

A useful hypothesis will enable predictions to be made, by deductive reasoning, that can be experimentally assessed. If results contradictory to the predictions are found, the hypothesis under test is incorrect or incomplete, requiring either revision or abandonment. If results confirming the hypothesis are found, the hypothesis might be correct, but is always subject to further test.

Einstein's theory of General Relativity makes several specific predictions about the observable structure of space-time, such as a prediction that light bends in a gravitational field, and that the amount of bending depends in a precise way on the strength of the gravitational field. Observations made during a 1919 solar eclipse supported General Relativity rather than Newtonian gravitation.


Once a prediction is made, an experiment is designed to test it. The experiment may seek either confirmation or falsification of the hypothesis.

Scientists assume an attitude of openness and accountability on the part of those conducting an experiment. Detailed recordkeeping is essential, to aid in recording and reporting on the experimental results, and providing evidence of the effectiveness and integrity of the procedure. They will also assist in reproducing the experimental results.

Integrity may be augmented by the introduction of a control. Two virtually identical experiments are run, in only one of which the factor being tested is varied. This serves to further isolate any causal phenomena. For example in testing a drug it is important to carefully test that the supposed effect of the drug is produced only by the drug itself. Doctors may do this with a double-blind study: two virtually identical groups of patients are compared, one of which receives the drug and one of which receives a placebo. Neither the patients nor the doctor know who is getting the real drug, isolating its effects.

Once the experiment is complete, the researcher determines whether the results (or data) gathered are what was predicted. If the experimental conclusions fail to match the predictions/hypothesis, then one returns to the failed hypothesis and re-iterates the process. If the experiment(s) appears "successful" - i.e. fits the hypothesis - then the results are to be published in a way which allows others (in theory) to reproduce the same experiments and results.

Evaluation and iteration

Testing and improvement

The scientific process is iterative. At any stage it is possible that some consideration will lead the scientist to repeat an earlier part of the process. Failure to develop an interesting hypothesis may lead a scientist to re-define the subject they are considering. Failure of a hypothesis to produce interesting and testable predictions may lead to reconsideration of the hypothesis or of the definition of the subject. Failure of the experiment to produce interesting results may lead the scientist to reconsidering the experimental method, the hypothesis or the definition of the subject.


Science is a social enterprise, and scientific work will become accepted by the community only if they can be verified. Crucially, experimental and theoretical results must be reproduced by others within the science community.

Peer review evaluation
Scientific journals use a process of peer review, in which scientists' papers describing experimental results and their conclusions are submitted to a panel of fellow scientists for evaluation.

Reproducibility is straightforward in simple cases — in proofs in theoretical physics, or in the chemical analysis of a salt, for example, where materials and techniques are readily accessible. Results that are not easily reproduced are often controversial. For example, the cold fusion experiments of Fleischmann and Pons were never peer reviewed — they were announced directly to the press before any other scientists were able to evaluate their efforts or reproduce their results. Their results have not been reproduced elsewhere in the decades since; the press announcement was regarded at the time, by most nuclear physicists, as very likely wrong. Peer review may well have turned up problems and led to a closer examination of the experimental evidence Fleischmann, Pons, et al. believed they had found. Paul Kammerer's experiments on acquired physical traits in amphibians (described in Arthur Koestler's The Midwife Toad) seem to have been deliberately faked, while the confusion in the 60s and 70s about 'polywater' seems to have been the result of microcontamination. Much embarrassment and wasted effort might have been avoided by proper peer review in many such cases.

On the other hand, peer review of new discoveries is sometimes not very open-minded. The proposed existence of infectious protein particles by Stanley Prusiner in 1982 caused much scoffing and even hostility since it seemingly contradicted the central dogma of molecular biology, yet he won the 1997 Nobel Prize in physiology or medicine for the discovery of prions. Similarly, dark matter wasn't believed when first theorized in 1933 by Fritz Zwicky, nor when observationally confirmed by Vera Rubin in 1978, but after multiple independent confirmations, dark matter is now accepted in astrophysics.


All scientific knowledge is in a state of flux, for at any time new evidence could be presented that contradicts a long-held hypothesis. A particularly luminous example is the theory of light. Light had long been supposed to be made of particles. Isaac Newton, and before him many of the Classical Greeks, was convinced it was so, but his light-is-particles account was overturned by evidence in favor of a wave theory of light suggested most notably in the early 1800s by Thomas Young an English physician. Light as waves neatly explained the observed diffraction and interference of light when, to the contrary, the light-as-a-particle theory did not. The wave interpretation of light was widely held to be unassailably correct for most of the 19th century. Around the turn of the century, however, observations were made that a wave theory of light could not explain. This new set of observations could be accounted for by Max Planck's quantum theory (including the photoelectric effect and Brownian motion—both from Albert Einstein), but not by a wave theory of light. Nor, for that matter, by the particle theory.

Evidence and assumptions

Evidence comes in different forms and quality, mostly due to underlying assumptions. An underlying assumption that 'objects heavier than air fall to the ground when dropped' is not likely to incite much disagreement. An underlying assumption like 'aliens abduct humans' however is an extraordinary claim which requires extraordinary proof. Most extraordinary claims also do not survive Occam's razor.

Elegance of hypothesis

In evaluating a hypothesis, scientists tend to look for theories that are "elegant" or "beautiful". In contrast to the usual English use of these terms, scientists have more specific meanings in mind. "Elegance" (or "beauty") refers to the ability of a theory to neatly explain as many of the known facts as possible, as simply as possible, or at least in a manner consistent with Occam's Razor while at the same time being aesthetically pleasing.

Philosophical issues

The study of the scientific method is distinct from the practice of science and is more a part of the philosophy, history and sociology of science than of science itself. Such studies have limited direct impact on day-to-day scientific practice.

The material presented below is intended to show that, as with all philosophical topics, some of the issues surrounding the scientific method are neither straightforward nor simple.

Theory-dependence of observation

The scientific method depends on observation, in defining the subject under investigation and in performing experiments.

Observation involves perception, and so is a cognitive process. That is, one does not make an observation passively, but is actively involved in distinguishing the thing being observed from surrounding sensory data. Therefore, observations depend on some underlying understanding of the way in which the world functions, and that understanding may influence what is perceived, noticed, or deemed worthy of consideration. (See the Sapir-Whorf Hypothesis for an early version of this understanding of the impact of cultural artifacts on our perceptions of the world.)

Empirical observation is supposedly used to determine the acceptability of some hypothesis within a theory. When someone claims to have made an observation, it is reasonable to ask them to justify their claim. Such a justification must itself make reference to the theory - operational definitions and hypotheses - in which the observation is embedded. That is, the observation is a component of the theory that also contains the hypothesis it either verifies or falsifies. But this means that the observation cannot serve as a neutral arbiter between competing hypotheses. Observation could only do this "neutrally" if it were independent of the theory.

Thomas Kuhn denied that it is ever possible to isolate the theory being tested from the influence of the theory in which the observations are grounded. He argued that observations always rely on a specific paradigm, and that it is not possible to evaluate competing paradigms independently. By "paradigm" he meant, essentially, a logically consistent "portrait" of the world, one that involves no logical contradictions. More than one such logically consistent construct can each paint a usable likeness of the world, but it is pointless to pit them against each other, theory against theory. Neither is a standard by which the other can be judged. Instead, the question is which "portrait" is judged by some set of people to promise the most in terms of “puzzle solving”.

For Kuhn, the choice of paradigm was sustained by, but not ultimately determined by, logical processes. The individual's choice between paradigms involves setting two or more “portraits" against the world and deciding which likeness is most promising. In the case of a general acceptance of one paradigm or another, Kuhn believed that it represented the consensus of the community of scientists. Acceptance or rejection of some paradigm is, he argued, more a social than a logical process.

That observation is embedded in theory does not mean that observations are irrelevant to science. Scientific understanding derives from observation, but the acceptance of scientific statements is dependent on the related theoretical background or paradigm a well as on observation. Coherentism and scepticism offer alternatives to foundationalism for dealing with the difficulty of grounding scientific theories in something more than observations.

Indeterminacy of theory under empirical test

The Quine-Duhem thesis points out that any theory can be made compatible with any empirical observation by the addition of suitable ad hoc hypotheses. This is analogous to the way in which an infinite number of curves can be drawn through any set of data points on a graph.

This thesis was accepted by Karl Popper, leading him to reject naïve falsification in favour of 'survival of the fittest', or most falsifiable, of scientific theories. In Popper's view, any hypothesis that does not make testable predictions is simply not science. Such a hypothesis may be useful or valuable, but it cannot be said to be science. Confirmation holism, developed by W. V. Quine, states that empirical data is not sufficient to make a judgment between theories. A theory can always be made to fit with the empirical data available.

That empirical evidence does not serve to determine between alternate theories does not imply that all theories are of equal value. Rather than pretending to use a universally applicable methodological principle, the scientist is making a personal choice when she chooses some particular theory over another.

One result of this is that specialists in the philosophy of science stress the requirement that observations made for the purposes of science be restricted to intersubjective objects. That is, science is restricted to those areas where there is general agreement on the nature of the observations involved. It is comparatively easy for folk to agree on observations of physical phenomena, harder for them to agree on observations of socia; or mental phenomena, and difficult in the extreme to reach agreement on matters of theology or ethics.


Scientific Method is often touted as determining which disciplines are scientific and which are not. Those which follow the scientific method might be considered sciences; those that do not are not. That is, method might be used as the criterion for demarcation between science and non-science.

If observation cannot act as a theory-independent foundation for the scientific enterprise, science becomes a cycle of hypothesising and verification embedded in a theoretical framework and tied to the 'real world' by the agreement of the scientific community. Popper's claim that only falsifiable statements are scientific does not help here (see The Criterion of Demarcation). The Quine-Duhem thesis argues that it is not possible to prove that a statement is falsified; rather, falsification occurs when the scientific community agrees that a statement is falsified.

Assuming this to be true, it is not obvious how scientific debate differs in any logical way from the debates of, for example, historians. Both work within a cycle of hypothesising and verification, historians by reference to historical documents (the past), scientists by reference to the experiments they propose to construct (the future).

One might argue that science occupies a special place because its experiments can be repeated, but using repetition as a demarcation criterion would disenfranchise areas that are at present considered to be science, such as palaeontology and cosmology.

Alternately, Kuhn claims that the explanatory success of science is explained by the way in which scientists are restricted to working within a particular paradigm.

Paul Feyerabend takes these arguments to their limit, arguing that science does not occupy a special place in terms of either its logic or method, and so that any claim to special authority made by scientists cannot be upheld. This leads to a particularly democratic and anarchist approach to knowledge formation.

Science as a communal activity

In his book The Structure of Scientific Revolutions Kuhn argues that the process of observation and evaluation take place within a paradigm. 'A paradigm is what the members of a community of scientists share, and, conversely, a scientific community consists of men who share a paradigm' (postscript, part 1). On this account, science can be done only as a part of a community, and is inherently a communal activity.

For Kuhn the fundamental difference between science and other disciplines is in the way in which the communities function. Others, especially Feyerabend and some post-modernist thinkers, have argued that there is insufficient difference between social practices in science and other disciplines to maintain this distinction. It is apparent that social factors play an important and direct role in scientific method, but that they do not serve to differentiate science from other disciplines. Furthermore, although on this account science is socially constructed, it does not follow that reality itself is a social construct. Kuhn’s ideas are equally applicable to both realist and anti-realist ontologies.

The scientific method is a source of ongoing debate and contention, and this area of study is undergoing considerable change. It appears that positivist, empiricists and falsificationist theories are unable to satisfy their aim of giving a definitive account of the logic of science. It may also be that the sociology of science is incapable of accounting for the success of the scientific enterprise.

Scientific method and the practice of science

The primary constraints on science are:

It has not always been like this: in the old days of the "gentleman scientist" funding (and to a lesser extent publication) were far weaker constraints.

Both of these constraints indirectly bring in the scientific method - work that too obviously violates the constraints will be difficult to publish and difficult to get funded. Journals do not require submitted papers to conform to anything more specific that "good scientific practice" and this is mostly enforced by peer review. Originality, importance and interest are more important - see for example the author guidelines for Nature.

Annotated list of related issues

Empirical methods Paradigm change The problem of induction questions the logical ground for induction as a basis for science. Scientific creativity When Method goes wrong Those interested in the scientific method can monitor changes to related pages by clicking on in the sidebar.

External links