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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."
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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
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.
Literary Hub, 28 Oct. 2025
Fan theories have swirled online about Byler, and not just in Schnapp’s comment section, but in discussion forums like Reddit.
—Angel Saunders, PEOPLE, 28 Oct. 2025
Plus, the actress shares her thoughts on that theory about Gladys being named for Elvis' mom, and more.
—Gerrad Hall, Entertainment Weekly, 27 Oct. 2025
The leading theory behind the moon’s formation suggests that in the earliest days of our solar system, baby Earth was flying solo until, after some tens of millions of years, a Mars-size object collided with it and sent debris flying into space.
—Lila Shroff, The Atlantic, 27 Oct. 2025
See All Example Sentences for theory
Cite this Entry
“Theory.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/theory. Accessed 30 Oct. 2025.
Copy CitationKids Definition
theory
noun
from Latin theoria "a looking at or considering of facts, theory," from Greek theōria "theory, action of viewing, consideration," from theōrein "to look at, consider," — related to theater
Medical Definition
theory
noun
ˈthi(-ə)r-ē
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