In ordinary speech, a theory is just a speculation. The police inspector in a Miss Marple mystery always has a theory about who committed the murder which turns out to be wrong.
But in science the word “theory” plays a very different role. What most of us call “theories” are termed “hypotheses” until enough evidence has been accumulated to validate them and allow them to assume the status of theories: scientifically acceptable explanations of phenomena. Examples: the theory of gravity, the wave theory of light, chaos theory.
Foes of evolutionary science often insist that the theory of evolution is invalid because it is “only a theory.” This merely demonstrates their lack of knowledge of scientific usage and hence will not impress any scientifically literate person.
Listen to Paul Brians discuss the meaning of “theory” on his Common Errors in English Podcast.
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