seeing is believing

Seeing is leeving.
[1609 S. Harward MS (Trinity College, Cambridge) 85]

Seeing is beleeving.
[1639 J. Clarke Parœmiologia Anglo-Latina 90]

There's nothing like Matter of Fact; Seeing is Believing.
[1712 J. Arbuthnot Lewis Baboon iv.]

Seeing is believing, says the proverb. ‥Though, of all our senses, the eyes are the most easily deceived, we believe them in preference to any other evidence.
[1848 J. C. & A. W. Hare Guesses at Truth (ed. 2) 2nd Ser. 497]

Seeing, as the old saw goes, is believing, and in the post-literate age the visual is more persuasive than it used to be.
[2001 Washington Times 14 Dec. A4]

Related to: trust and scepticism

Bibliography of major proverb collections and works cited from modern editions is available here.

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.