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Phrase(s): Seeing is believing.
Prov. It is hard to believe something you have not seen. (Implies that you will not believe the thing under discussion until you have actually seen it.) • Jill: They say Melissa has become a wonderful housekeeper now that she has her own apartment. Jane: Seeing is believing. • I really didn’t think that Jerry’s girlfriend could be as pretty as he said she was, but seeing is believing.
McGrawhill's American Idioms And Phrasal Verbs
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Only physical or concrete evidence is convincing, as in She wrote us that she's lost twenty pounds, but seeing is believing. This idiom was first recorded in this form in 1639.
American Heritage Idioms