“If you believe that you will believe anything.”
In reply to a man who greeted him in the street with the words "Mr. Jones, I believe?", as quoted in Wellington — The Years of the Sword (1969) by Elizabeth Longford.
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Arthur Wellesley, 1st Duke of Wellington 45
British soldier and statesman 1769–1852Related quotes

“I don't believe in anything you have to believe in.”
Reported by Steve Pinker, " I am Steve Pinker, a cognitive psychologist at Harvard. Ask me anything. http://www.reddit.com/r/IAmA/comments/1a67x4/i_am_steve_pinker_a_cognitive_psychologist_at/c8uiccn", Reddit.com (March 12, 2013).
Other

“If you believe in yourself, anything is possible.”

“You can blame a wizard for anything and people will believe you.”
Source: Bleak Seasons (1996), Chapter 77 (p. 212)

“Believe in yourself and you can accomplish anything.”
Wake-up words on Vine https://vine.co/v/OrYTHrW7hWP, 2014

Sometimes attributed to Contact (1985), but the quote does not appear in that book.
It appears attributed to Sagan in Judson Poling's 2003 book "Do Science and the Bible Conflict?", but without source.
Misattributed
Source: Via Google Books https://books.google.com/books?id=Ondv5WzhYYYC&pg=PA21&dq=Sagan
“You can make an audience see nearly anything, if you yourself believe in it.”
The Mask of Apollo (1966)

“The truth you believe and cling to makes you unavailable to hear anything new.”