theory

noun
the·​o·​ry | \ˈthē-ə-rē, ˈthir-ē\
plural theories

Definition of theory 

1 : a plausible or scientifically acceptable general principle or body of principles offered to explain phenomena the wave theory of light

2a : a belief, policy, or procedure proposed or followed as the basis of action her method is based on the theory that all children want to learn

b : an ideal or hypothetical set of facts, principles, or circumstances often used in the phrase in theory in theory, we have always advocated freedom for all

3a : a hypothesis assumed for the sake of argument or investigation

b : an unproved assumption : conjecture

c : a body of theorems presenting a concise systematic view of a subject theory of equations

4 : the general or abstract principles of a body of fact, a science, or an art music theory

5 : abstract thought : speculation

6 : the analysis of a set of facts in their relation to one another

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Choose the Right Synonym for theory

hypothesis, theory, law mean a formula derived by inference from scientific data that explains a principle operating in nature. hypothesis implies insufficient evidence to provide more than a tentative explanation. a hypothesis explaining the extinction of the dinosaurs theory implies a greater range of evidence and greater likelihood of truth. the theory of evolution law implies a statement of order and relation in nature that has been found to be invariable under the same conditions. the law of gravitation

The Difference Between Hypothesis and Theory

A hypothesis is an assumption, an idea that is proposed for the sake of argument so that it can be tested to see if it might be true.

In the scientific method, the hypothesis is constructed before any applicable research has been done, apart from a basic background review. You ask a question, read up on what has been studied before, and then form a hypothesis.

A hypothesis is usually tentative; it's an assumption or suggestion made strictly for the objective of being tested.

A theory, in contrast, is a principle that has been formed as an attempt to explain things that have already been substantiated by data. It is used in the names of a number of principles accepted in the scientific community, such as the Big Bang Theory. Because of the rigors of experimentation and control, it is understood to be more likely to be true than a hypothesis is.

In non-scientific use, however, hypothesis and theory are often used interchangeably to mean simply an idea, speculation, or hunch, with theory being the more common choice.

Since this casual use does away with the distinctions upheld by the scientific community, hypothesis and theory are prone to being wrongly interpreted even when they are encountered in scientific contexts—or at least, contexts that allude to scientific study without making the critical distinction that scientists employ when weighing hypotheses and theories.

The most common occurrence is when theory is interpreted—and sometimes even gleefully seized upon—to mean something having less truth value than other scientific principles. (The word law applies to principles so firmly established that they are almost never questioned, such as the law of gravity.)

This mistake is one of projection: since we use theory in general to mean something lightly speculated, then it's implied that scientists must be talking about the same level of uncertainty when they use theory to refer to their well-tested and reasoned principles.

The distinction has come to the forefront particularly on occasions when the content of science curricula in schools has been challenged—notably, when a school board in Georgia put stickers on textbooks stating that evolution was "a theory, not a fact, regarding the origin of living things." As Kenneth R. Miller, a cell biologist at Brown University, has said, a theory "doesn’t mean a hunch or a guess. A theory is a system of explanations that ties together a whole bunch of facts. It not only explains those facts, but predicts what you ought to find from other observations and experiments.”

While theories are never completely infallible, they form the basis of scientific reasoning because, as Miller said "to the best of our ability, we’ve tested them, and they’ve held up."

Two Related, Yet Distinct, Meanings of Theory

There are many shades of meaning to the word theory. Most of these are used without difficulty, and we understand, based on the context in which they are found, what the intended meaning is. For instance, when we speak of music theory we understand it to be in reference to the underlying principles of the composition of music, and not in reference to some speculation about those principles.

However, there are two senses of theory which are sometimes troublesome. These are the senses which are defined as “a plausible or scientifically acceptable general principle or body of principles offered to explain phenomena” and “an unproven assumption; conjecture.” The second of these is occasionally misapplied in cases where the former is meant, as when a particular scientific theory is derided as "just a theory," implying that it is no more than speculation or conjecture. One may certainly disagree with scientists regarding their theories, but it is an inaccurate interpretation of language to regard their use of the word as implying a tentative hypothesis; the scientific use of theory is quite different than the speculative use of the word.

Examples of theory in a Sentence

The immune surveillance theory of cancer holds that in a way we all do have cancer, that a healthy immune system fights off rogue cells as they appear. — Sallie Tisdale, Harper's, June 2007 The family's theory was that the cheating businessmen somehow framed their brother. — Eliza Griswold, Harper's, September 2006 The theory of the teacher with all these immigrant kids was that if you spoke English loudly enough they would eventually understand. — E. L. Doctorow, Loon Lake, (1979) 1980 While strolling around, we kept the run of the moon all the time, and we still kept an eye on her after we got back to the hotel portico. I had a theory that the gravitation of refraction, being subsidiary to atmospheric compensation, the refrangibility of the earth's surface would emphasize this effect in regions where great mountain ranges occur, and possibly so even-handed impact the odic and idyllic forces together, the one upon the other, as to prevent the moon from rising higher than 12,200 feet above sea-level. This daring theory had been received with frantic scorn by some of my fellow-scientists, and with an eager silence by others. — Mark Twain, A Tramp Abroad, 1880 a widely accepted scientific theory Her method is based on the theory that all children want to learn. There are a number of different theories about the cause of the disease. She proposed a theory of her own. Investigators rejected the theory that the death was accidental. There is no evidence to support such a theory. He is a specialist in film theory and criticism.
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Recent Examples on the Web

Our electron, according to theory, is actually a football with charge at the point ends. Chris Lee, Ars Technica, "Physicists report electron is round—what does that mean?," 7 Nov. 2018 However, sometimes astronomers will write up wild theories like this, so that those in the community can dissect the claim and pick it apart. Loren Grush, The Verge, "Why we shouldn’t cry ‘aliens’ about that interstellar space rock just yet," 6 Nov. 2018 The resulting theory, quantum mechanics, deals with particles and blips of energy in the realm of the ultra-small, divorced from our everyday experience, and all but invisible to our clumsy mammalian sensory apparatuses. Tim Folger, Discover Magazine, "How Quantum Mechanics Lets Us See, Smell and Touch," 24 Oct. 2018 There are theories that they might be involved with the planet's current rate of shrinking and its past volcanic activity, but nobody knows for sure. David Grossman, Popular Mechanics, "The BepiColombo Probe Is About to Being a 7-Year Mission to Mercury," 19 Oct. 2018 But Einstein’s theory made important predictions about black holes too, notably that a black hole can be completely defined by only three features: its mass, charge, and spin. Fox News, "Video from 2011 creates headaches for Sinema," 12 Oct. 2018 Fan theories are fun, but man, do people take them seriously. Amanda Mitchell, Marie Claire, "Cole Sprouse Spills His Favorite 'Riverdale' Fan Theory in a Surprise Reddit AMA," 8 Oct. 2018 Feeling like this right about now: Then again, all three theories could be simultaneously correct ... who knows, honestly. Kayla Keegan, Good Housekeeping, "Here's What Franco Harris Might Have to Do With "This Is Us'," 26 Sep. 2018 Old theories of what’s wrong with the labor market — such as a lack of people with necessary skills — are dying fast. Andrew Van Dam, latimes.com, "The economy is hot, yet many U.S. workers feel left behind. A new report sheds some light," 5 July 2018

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'theory.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

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First Known Use of theory

1592, in the meaning defined at sense 6

History and Etymology for theory

Late Latin theoria, from Greek theōria, from theōrein

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Statistics for theory

Last Updated

15 Nov 2018

Look-up Popularity

Time Traveler for theory

The first known use of theory was in 1592

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More Definitions for theory

theory

noun

English Language Learners Definition of theory

: an idea or set of ideas that is intended to explain facts or events

: an idea that is suggested or presented as possibly true but that is not known or proven to be true

: the general principles or ideas that relate to a particular subject

theory

noun
the·​o·​ry | \ˈthē-ə-rē, ˈthir-ē\
plural theories

Kids Definition of theory

1 : an idea or opinion that is presented as true Nobody knows where he went, but each of us has a theory. Perhaps they were formulating their own theories about how Cedric had died.— J. K. Rowling, Goblet of Fire

2 : a general rule offered to explain a scientific phenomenon the theory of gravity

3 : the general rules followed in a science or an art music theory

theory

noun
the·​o·​ry | \ˈthē-ə-rē, ˈthi(-ə)r-ē \
plural theories

Medical Definition of theory 

1 : the general or abstract principles of a body of fact, a science, or an art the theory and practice of medicine

2 : a plausible or scientifically acceptable general principle or body of principles offered to explain natural phenomena a theory of organic evolution — see atomic theory, cell theory, germ theory

3 : a working hypothesis that is considered probable based on experimental evidence or factual or conceptual analysis and is accepted as a basis for experimentation

Other Words from theory

theoretical \ ˌthē-​ə-​ˈret-​i-​kəl, ˌthi(ə)r-​ˈet-​ \ also theoretic \ -​ik \ adjective
theoretically \ -​i-​k(ə-​)lē \ adverb

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More from Merriam-Webster on theory

Thesaurus: All synonyms and antonyms for theory

Spanish Central: Translation of theory

Nglish: Translation of theory for Spanish Speakers

Britannica English: Translation of theory for Arabic Speakers

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