meaning of "In theory"

[Lungo]

Test No. incompl/elem-27 "Sailing", question 6

In ......... I agree.

(a) idea
(b) philosophy
(c) theory
(d) belief

Test No. incompl/elem-27 "Sailing", answer 6

In theory I agree.

Correct answer: (c) theory
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what does theory mean?

Lungo

[Alan]

Hi Linh,

In theory means as an idea or theoretically. If you say to someone: I agree with what you say in theory, it suggests that you don't think the idea is practical.

Alan
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English as a Second Language
You can read my ESL story New year resolutions

[Zellzacks]

Hi Alan,

About your explanations, why we cannot use 'idea'? "In idea I agree"?

thank you