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A scientific theory is an explanation of an aspect of the natural
world that can be repeatedly tested, in accordance with the scientific
method, using a predefined protocol of observation and
experiment. Established scientific theories have withstood
rigorous scrutiny and embody scientific knowledge.
The definition of a scientific theory (often contracted to "theory"
for the sake of brevity) as used in the disciplines of science is
significantly different from the common vernacular usage of the word
"theory".[Note 1] In everyday speech, "theory" can imply that
something is an unsubstantiated and speculative guess, the opposite
of its meaning in science. These different usages are comparable to
the opposing usages of "prediction" in science versus everyday speech,
where it denotes a mere hope.
The strength of a scientific theory is related to the diversity of
phenomena it can explain and its simplicity. As additional scientific
evidence is gathered, a scientific theory may be modified and
ultimately rejected if it cannot be made to fit the new findings; in
such circumstances, a more accurate theory is then required. In
certain cases, the less-accurate unmodified scientific theory can
still be treated as a theory if it is useful (due to its sheer
simplicity) as an approximation under specific conditions. A case in
point is Newton's laws of motion, which can serve as an approximation
to special relativity at velocities that are small relative to the
speed of light.
Scientific theories are testable and make falsifiable predictions.
They describe the causes of a particular natural phenomenon and are
used to explain and predict aspects of the physical universe or
specific areas of inquiry (for example, electricity, chemistry, and
astronomy). Scientists use theories to further scientific knowledge,
as well as to facilitate advances in technology or medicine.
As with other forms of scientific knowledge, scientific theories are
both deductive and inductive, aiming for predictive and
Stephen Jay Gould
1 Types 2 Characteristics
2.1 Essential criteria 2.2 Other criteria 2.3 Definitions from scientific organizations
3 Formation 4 Modification and improvement
4.1 Unification 4.2 Example: Relativity
5 Theories and laws 6 About theories
6.1 Theories as axioms 6.2 Theories as models
6.2.1 Differences between theory and model
6.3 Assumptions in formulating theories
7.1 From philosophers of science 7.2 Analogies and metaphors
8 In physics 9 Examples 10 Further reading 11 References 12 Notes
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Typically for any theory to be accepted within most academia there is one simple criterion. The essential criterion is that the theory must be observable and repeatable. The aforementioned criterion is essential to prevent fraud and perpetuate science itself.
The tectonic plates of the world were mapped in the second half of the 20th century. Plate tectonic theory successfully explains numerous observations about the Earth, including the distribution of earthquakes, mountains, continents, and oceans.
The defining characteristic of all scientific knowledge, including theories, is the ability to make falsifiable or testable predictions. The relevance and specificity of those predictions determine how potentially useful the theory is. A would-be theory that makes no observable predictions is not a scientific theory at all. Predictions not sufficiently specific to be tested are similarly not useful. In both cases, the term "theory" is not applicable. A body of descriptions of knowledge can be called a theory if it fulfills the following criteria:
It makes falsifiable predictions with consistent accuracy across a broad area of scientific inquiry (such as mechanics). It is well-supported by many independent strands of evidence, rather than a single foundation. It is consistent with preexisting experimental results and at least as accurate in its predictions as are any preexisting theories.
These qualities are certainly true of such established theories as special and general relativity, quantum mechanics, plate tectonics, the modern evolutionary synthesis, etc. Other criteria In addition, scientists prefer to work with a theory that meets the following qualities:
It can be subjected to minor adaptations to account for new data that do not fit it perfectly, as they are discovered, thus increasing its predictive capability over time. It is among the most parsimonious explanations, economical in the use of proposed entities or explanatory steps as per Occam's razor. This is because for each accepted explanation of a phenomenon, there may be an extremely large, perhaps even incomprehensible, number of possible and more complex alternatives, because one can always burden failing explanations with ad hoc hypotheses to prevent them from being falsified; therefore, simpler theories are preferable to more complex ones because they are more testable.
Definitions from scientific organizations
United States National Academy of Sciences
The formal scientific definition of theory is quite different from the everyday meaning of the word. It refers to a comprehensive explanation of some aspect of nature that is supported by a vast body of evidence. Many scientific theories are so well established that no new evidence is likely to alter them substantially. For example, no new evidence will demonstrate that the Earth does not orbit around the sun (heliocentric theory), or that living things are not made of cells (cell theory), that matter is not composed of atoms, or that the surface of the Earth is not divided into solid plates that have moved over geological timescales (the theory of plate tectonics)...One of the most useful properties of scientific theories is that they can be used to make predictions about natural events or phenomena that have not yet been observed.
From the American Association for the Advancement of Science:
A scientific theory is a well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment. Such fact-supported theories are not "guesses" but reliable accounts of the real world. The theory of biological evolution is more than "just a theory". It is as factual an explanation of the universe as the atomic theory of matter or the germ theory of disease. Our understanding of gravity is still a work in progress. But the phenomenon of gravity, like evolution, is an accepted fact.
Note that the term theory would not be appropriate for describing untested but intricate hypotheses or even scientific models. Formation
The first observation of cells, by Robert Hooke, using an early microscope. This led to the development of cell theory.
The scientific method involves the proposal and testing of hypotheses, by deriving predictions from the hypotheses about the results of future experiments, then performing those experiments to see whether the predictions are valid. This provides evidence either for or against the hypothesis. When enough experimental results have been gathered in a particular area of inquiry, scientists may propose an explanatory framework that accounts for as many of these as possible. This explanation is also tested, and if it fulfills the necessary criteria (see above), then the explanation becomes a theory. This can take many years, as it can be difficult or complicated to gather sufficient evidence. Once all of the criteria have been met, it will be widely accepted by scientists (see scientific consensus) as the best available explanation of at least some phenomena. It will have made predictions of phenomena that previous theories could not explain or could not predict accurately, and it will have resisted attempts at falsification. The strength of the evidence is evaluated by the scientific community, and the most important experiments will have been replicated by multiple independent groups. Theories do not have to be perfectly accurate to be scientifically useful. For example, the predictions made by classical mechanics are known to be inaccurate in the relatistivic realm, but they are almost exactly correct at the comparatively low velocities of common human experience. In chemistry, there are many acid-base theories providing highly divergent explanations of the underlying nature of acidic and basic compounds, but they are very useful for predicting their chemical behavior. Like all knowledge in science, no theory can ever be completely certain, since it is possible that future experiments might conflict with the theory's predictions. However, theories supported by the scientific consensus have the highest level of certainty of any scientific knowledge; for example, that all objects are subject to gravity or that life on Earth evolved from a common ancestor. Acceptance of a theory does not require that all of its major predictions be tested, if it is already supported by sufficiently strong evidence. For example, certain tests may be unfeasible or technically difficult. As a result, theories may make predictions that have not yet been confirmed or proven incorrect; in this case, the predicted results may be described informally with the term "theoretical". These predictions can be tested at a later time, and if they are incorrect, this may lead to the revision or rejection of the theory. Modification and improvement If experimental results contrary to a theory's predictions are observed, scientists first evaluate whether the experimental design was sound, and if so they confirm the results by independent replication. A search for potential improvements to the theory then begins. Solutions may require minor or major changes to the theory, or none at all if a satisfactory explanation is found within the theory's existing framework. Over time, as successive modifications build on top of each other, theories consistently improve and greater predictive accuracy is achieved. Since each new version of a theory (or a completely new theory) must have more predictive and explanatory power than the last, scientific knowledge consistently becomes more accurate over time. If modifications to the theory or other explanations seem to be insufficient to account for the new results, then a new theory may be required. Since scientific knowledge is usually durable, this occurs much less commonly than modification. Furthermore, until such a theory is proposed and accepted, the previous theory will be retained. This is because it is still the best available explanation for many other phenomena, as verified by its predictive power in other contexts. For example, it has been known since 1859 that the observed perihelion precession of Mercury violates Newtonian mechanics, but the theory remained the best explanation available until relativity was supported by sufficient evidence. Also, while new theories may be proposed by a single person or by many, the cycle of modifications eventually incorporates contributions from many different scientists. After the changes, the accepted theory will explain more phenomena and have greater predictive power (if it did not, the changes would not be adopted); this new explanation will then be open to further replacement or modification. If a theory does not require modification despite repeated tests, this implies that the theory is very accurate. This also means that accepted theories continue to accumulate evidence over time, and the length of time that a theory (or any of its principles) remains accepted often indicates the strength of its supporting evidence. Unification
In quantum mechanics, the electrons of an atom occupy orbitals around the nucleus. This image shows the orbitals of a hydrogen atom (s, p, d) at three different energy levels (1, 2, 3). Brighter areas correspond to higher probability density.
In some cases, two or more theories may be replaced by a single theory
that explains the previous theories as approximations or special
cases, analogous to the way a theory is a unifying explanation for
many confirmed hypotheses; this is referred to as unification of
theories. For example, electricity and magnetism are now known to
be two aspects of the same phenomenon, referred to as
When the predictions of different theories appear to contradict each
other, this is also resolved by either further evidence or
unification. For example, physical theories in the 19th century
implied that the
Precession of the perihelion of Mercury (exaggerated). The deviation in Mercury's position from the Newtonian prediction is about 43 arc-seconds (about two-thirds of 1/60 of a degree) per century.
In this approach, theories are a specific category of models that
fulfill the necessary criteria (see above). One can use language to
describe a model; however, the theory is the model (or a collection of
similar models), and not the description of the model. A model of the
solar system, for example, might consist of abstract objects that
represent the sun and the planets. These objects have associated
properties, e.g., positions, velocities, and masses. The model
parameters, e.g., Newton's Law of Gravitation, determine how the
positions and velocities change with time. This model can then be
tested to see whether it accurately predicts future observations;
astronomers can verify that the positions of the model's objects over
time match the actual positions of the planets. For most planets, the
Newtonian model's predictions are accurate; for Mercury, it is
slightly inaccurate and the model of general relativity must be used
The word "semantic" refers to the way that a model represents the real
world. The representation (literally, "re-presentation") describes
particular aspects of a phenomenon or the manner of interaction among
a set of phenomena. For instance, a scale model of a house or of a
solar system is clearly not an actual house or an actual solar system;
the aspects of an actual house or an actual solar system represented
in a scale model are, only in certain limited ways, representative of
the actual entity. A scale model of a house is not a house; but to
someone who wants to learn about houses, analogous to a scientist who
wants to understand reality, a sufficiently detailed scale model may
Differences between theory and model
Main article: Conceptual model
Several commentators have stated that the distinguishing
characteristic of theories is that they are explanatory as well as
descriptive, while models are only descriptive (although still
predictive in a more limited sense). Philosopher
Stephen Pepper also
distinguished between theories and models, and said in 1948 that
general models and theories are predicated on a "root" metaphor that
constrains how scientists theorize and model a phenomenon and thus
arrive at testable hypotheses.
...it is incorrect to speak of an assumption as either true or false, since there is no way of proving it to be either (If there were, it would no longer be an assumption). It is better to consider assumptions as either useful or useless, depending on whether deductions made from them corresponded to reality...Since we must start somewhere, we must have assumptions, but at least let us have as few assumptions as possible.
Certain assumptions are necessary for all empirical claims (e.g. the
assumption that reality exists). However, theories do not generally
make assumptions in the conventional sense (statements accepted
without evidence). While assumptions are often incorporated during the
formation of new theories, these are either supported by evidence
(such as from previously existing theories) or the evidence is
produced in the course of validating the theory. This may be as simple
as observing that the theory makes accurate predictions, which is
evidence that any assumptions made at the outset are correct or
approximately correct under the conditions tested.
Conventional assumptions, without evidence, may be used if the theory
is only intended to apply when the assumption is valid (or
approximately valid). For example, the special theory of relativity
assumes an inertial frame of reference. The theory makes accurate
predictions when the assumption is valid, and does not make accurate
predictions when the assumption is not valid. Such assumptions are
often the point with which older theories are succeeded by new ones
(the general theory of relativity works in non-inertial reference
frames as well).
The term "assumption" is actually broader than its standard use,
etymologically speaking. The Oxford English Dictionary (OED) and
online Wiktionary indicate its Latin source as assumere ("accept, to
take to oneself, adopt, usurp"), which is a conjunction of ad- ("to,
towards, at") and sumere (to take). The root survives, with shifted
meanings, in the Italian sumere and Spanish sumir. The first sense of
"assume" in the OED is "to take unto (oneself), receive, accept,
adopt". The term was originally employed in religious contexts as in
"to receive up into heaven", especially "the reception of the Virgin
Mary into heaven, with body preserved from corruption", (1297 CE) but
it was also simply used to refer to "receive into association" or
"adopt into partnership". Moreover, other senses of assumere included
(i) "investing oneself with (an attribute)", (ii) "to undertake"
(especially in Law), (iii) "to take to oneself in appearance only, to
pretend to possess", and (iv) "to suppose a thing to be" (all senses
from OED entry on "assume"; the OED entry for "assumption" is almost
perfectly symmetrical in senses). Thus, "assumption" connotes other
associations than the contemporary standard sense of "that which is
assumed or taken for granted; a supposition, postulate" (only the 11th
of 12 senses of "assumption", and the 10th of 11 senses of "assume").
From philosophers of science
It is easy to obtain confirmations, or verifications, for nearly every theory—if we look for confirmations. Confirmations should count only if they are the result of risky predictions; that is to say, if, unenlightened by the theory in question, we should have expected an event which was incompatible with the theory—an event which would have refuted the theory. Every "good" scientific theory is a prohibition: it forbids certain things to happen. The more a theory forbids, the better it is. A theory which is not refutable by any conceivable event is non-scientific. Irrefutability is not a virtue of a theory (as people often think) but a vice. Every genuine test of a theory is an attempt to falsify it, or to refute it. Testability is falsifiability; but there are degrees of testability: some theories are more testable, more exposed to refutation, than others; they take, as it were, greater risks. Confirming evidence should not count except when it is the result of a genuine test of the theory; and this means that it can be presented as a serious but unsuccessful attempt to falsify the theory. (I now speak in such cases of "corroborating evidence".) Some genuinely testable theories, when found to be false, might still be upheld by their admirers—for example by introducing post hoc (after the fact) some auxiliary hypothesis or assumption, or by reinterpreting the theory post hoc in such a way that it escapes refutation. Such a procedure is always possible, but it rescues the theory from refutation only at the price of destroying, or at least lowering, its scientific status, by tampering with evidence. The temptation to tamper can be minimized by first taking the time to write down the testing protocol before embarking on the scientific work.
Popper summarized these statements by saying that the central
criterion of the scientific status of a theory is its "falsifiability,
or refutability, or testability". Echoing this, Stephen Hawking
states, "A theory is a good theory if it satisfies two requirements:
It must accurately describe a large class of observations on the basis
of a model that contains only a few arbitrary elements, and it must
make definite predictions about the results of future observations."
He also discusses the "unprovable but falsifiable" nature of theories,
which is a necessary consequence of inductive logic, and that "you can
disprove a theory by finding even a single observation that disagrees
with the predictions of the theory".
Several philosophers and historians of science have, however, argued
that Popper's definition of theory as a set of falsifiable statements
is wrong because, as
Philip Kitcher has pointed out, if one took a
strictly Popperian view of "theory", observations of
Unity: "A science should be unified…. Good theories consist of just one problem-solving strategy, or a small family of problem-solving strategies, that can be applied to a wide range of problems." Fecundity: "A great scientific theory, like Newton's, opens up new areas of research…. Because a theory presents a new way of looking at the world, it can lead us to ask new questions, and so to embark on new and fruitful lines of inquiry…. Typically, a flourishing science is incomplete. At any time, it raises more questions than it can currently answer. But incompleteness is not vice. On the contrary, incompleteness is the mother of fecundity…. A good theory should be productive; it should raise new questions and presume those questions can be answered without giving up its problem-solving strategies." Auxiliary hypotheses that are independently testable: "An auxiliary hypothesis ought to be testable independently of the particular problem it is introduced to solve, independently of the theory it is designed to save." (For example, the evidence for the existence of Neptune is independent of the anomalies in Uranus's orbit.)
Like other definitions of theories, including Popper's, Kitcher makes it clear that a theory must include statements that have observational consequences. But, like the observation of irregularities in the orbit of Uranus, falsification is only one possible consequence of observation. The production of new hypotheses is another possible and equally important result. Analogies and metaphors The concept of a scientific theory has also been described using analogies and metaphors. For instance, the logical empiricist Carl Gustav Hempel likened the structure of a scientific theory to a "complex spatial network:"
Its terms are represented by the knots, while the threads connecting the latter correspond, in part, to the definitions and, in part, to the fundamental and derivative hypotheses included in the theory. The whole system floats, as it were, above the plane of observation and is anchored to it by the rules of interpretation. These might be viewed as strings which are not part of the network but link certain points of the latter with specific places in the plane of observation. By virtue of these interpretive connections, the network can function as a scientific theory: From certain observational data, we may ascend, via an interpretive string, to some point in the theoretical network, thence proceed, via definitions and hypotheses, to other points, from which another interpretive string permits a descent to the plane of observation.
A theory is something other than myself. It may be set out on paper as a system of rules, and it is the more truly a theory the more completely it can be put down in such terms. Mathematical theory reaches the highest perfection in this respect. But even a geographical map fully embodies in itself a set of strict rules for finding one's way through a region of otherwise uncharted experience. Indeed, all theory may be regarded as a kind of map extended over space and time.
A scientific theory can also be thought of as a book that captures the
fundamental information about the world, a book that must be
researched, written, and shared. In 1623,
Philosophy [i.e. physics] is written in this grand book—I mean the universe—which stands continually open to our gaze, but it cannot be understood unless one first learns to comprehend the language and interpret the characters in which it is written. It is written in the language of mathematics, and its characters are triangles, circles, and other geometrical figures, without which it is humanly impossible to understand a single word of it; without these, one is wandering around in a dark labyrinth.
The book metaphor could also be applied in the following passage, by the contemporary philosopher of science Ian Hacking:
I myself prefer an Argentine fantasy. God did not write a Book of Nature of the sort that the old Europeans imagined. He wrote a Borgesian library, each book of which is as brief as possible, yet each book of which is inconsistent with every other. No book is redundant. For every book there is some humanly accessible bit of Nature such that that book, and no other, makes possible the comprehension, prediction and influencing of what is going on…Leibniz said that God chose a world which maximized the variety of phenomena while choosing the simplest laws. Exactly so: but the best way to maximize phenomena and have simplest laws is to have the laws inconsistent with each other, each applying to this or that but none applying to all.
In physics In physics, the term theory is generally used for a mathematical framework—derived from a small set of basic postulates (usually symmetries—like equality of locations in space or in time, or identity of electrons, etc.)—that is capable of producing experimental predictions for a given category of physical systems. A good example is classical electromagnetism, which encompasses results derived from gauge symmetry (sometimes called gauge invariance) in a form of a few equations called Maxwell's equations. The specific mathematical aspects of classical electromagnetic theory are termed "laws of electromagnetism," reflecting the level of consistent and reproducible evidence that supports them. Within electromagnetic theory generally, there are numerous hypotheses about how electromagnetism applies to specific situations. Many of these hypotheses are already considered to be adequately tested, with new ones always in the making and perhaps untested. An example of the latter might be the radiation reaction force. As of 2009, its effects on the periodic motion of charges are detectable in synchrotrons, but only as averaged effects over time. Some researchers are now considering experiments that could observe these effects at the instantaneous level (i.e. not averaged over time). Examples Note that many fields of inquiry do not have specific named theories, e.g. developmental biology. Scientific knowledge outside a named theory can still have a high level of certainty, depending on the amount of evidence supporting it. Also note that since theories draw evidence from many different fields, the categorization is not absolute.
Biology: cell theory, theory of evolution (modern evolutionary
synthesis), germ theory, particulate inheritance theory, dual
Chemistry: collision theory, kinetic theory of gases, Lewis theory,
molecular theory, molecular orbital theory, transition state theory,
valence bond theory
Physics: atomic theory,
Sellers, Piers (August 17, 2016). "Space, Climate Change, and the Real Meaning of Theory". The New Yorker. Retrieved August 18, 2016. , essay by British/American meteorologist and a NASA astronaut on anthopogenic global warming and "theory"
^ Per NAS 2008: "The formal scientific definition of theory is quite different from the everyday meaning of the word. It refers to a comprehensive explanation of some aspect of nature that is supported by a vast body of evidence."
^ National Academy of Sciences, 1999
^ "The Structure of Scientific Theories" in The Stanford Encyclopedia
^ Schafersman, Steven D. "An Introduction to Science".
^ a b c National Academy of Sciences, 2008.
^ a b c Popper, Karl (1963), Conjectures and Refutations, Routledge
and Kegan Paul, London, UK. Reprinted in
Theodore Schick (ed., 2000),
Readings in the Philosophy of Science, Mayfield Publishing Company,
Mountain View, Calif.
^ Andersen, Hanne; Hepburn, Brian (2015). Edward N. Zalta, ed.
Scientific Method. The Stanford Encyclopedia of Philosophy. CS1
maint: Multiple names: authors list (link)
^ The Devil in Dover, p. 98
^ Alan Baker (2010) . "Simplicity". Stanford Encyclopedia of
Philosophy. California: Stanford University.
^ Courtney A, Courtney M (2008). "Comments Regarding "On the Nature Of
v t e
Philosophy of science
A priori and a posteriori
Ignoramus et ignorabimus
Problem of induction
Metatheory of science
Coherentism Confirmation holism Constructive empiricism Constructive realism Constructivist epistemology Contextualism Conventionalism Deductive-nomological model Hypothetico-deductive model Inductionism Epistemological anarchism Evolutionism Fallibilism Foundationalism Instrumentalism Pragmatism Model-dependent realism Naturalism Physicalism Positivism / Reductionism / Determinism Rationalism / Empiricism Received view / Semantic view of theories Scientific realism / Anti-realism Scientific essentialism Scientific formalism Scientific skepticism Scientism Structuralism Uniformitarianism Vitalism
thermal and statistical Motion
Chemistry Biology Environment Geography Social science Technology
Engineering Artificial intelligence Computer science
Information Mind Psychiatry Psychology Perception Space and time
Criticism of science
Faith and rationality
Philosophers of science by era
Plato Aristotle Stoicism Epicureans
Averroes Avicenna Roger Bacon William of Ockham Hugh of Saint Victor Dominicus Gundissalinus Robert Kilwardby
Francis Bacon Thomas Hobbes René Descartes Galileo Galilei Pierre Gassendi Isaac Newton David Hume
Immanuel Kant Friedrich Schelling William Whewell Auguste Comte John Stuart Mill Herbert Spencer Wilhelm Wundt Charles Sanders Peirce Wilhelm Windelband Henri Poincaré Pierre Duhem Rudolf Steiner Karl Pearson
Alfred North Whitehead Bertrand Russell Albert Einstein Otto Neurath C. D. Broad Michael Polanyi Hans Reichenbach Rudolf Carnap Karl Popper Carl Gustav Hempel W. V. O. Quine Thomas Kuhn Imre Lakatos Paul Feyerabend Jürgen Habermas Ian Hacking Bas van Fraassen Larry Laudan Daniel Dennett