A theory is a set of ideas that explains something, often supported by evidence.

The idea

Use countable theory: a theory, two theories. A theory of something identifies its subject. In theory means according to an idea, before practical results are known.

Examples

  • The scientist proposed a theory of how stars form.
  • New evidence may challenge the theory.
  • The students compared two theories of language learning.
  • Her theory explains why the river changed course.
  • The team tested the theory with data from several cities.
  • In theory, the plan should save time, but we have not tried it.

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Watch out

Do not assume a scientific theory is merely a guess: it can be a well-supported explanation.

Try it yourself

  1. Choose the correct option. The researcher developed a ___ of how memories form. (theory / theories)

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    theory

    Theory follows the singular article a.

  2. Choose the correct option. The class discussed several ___ about social change. (theory / theories)

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    theories

    Theories is plural after several.

  3. Choose the correct option. The idea works ___ theory, but it has not been tested. (in / on)

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    in

    In completes the expression for what is expected before practical testing.

Use it today

Describe a theory and say how someone could test it.