Prejudice, not being founded on reason, cannot be removed by argument.

Samuel Johnson (1709-1784) English writer, lexicographer, critic

(Spurious)
    (Source)

Frequently attributed without citation, and not found in Johnson's works.  However, the phrase can be found in other contexts:

• "This objection on the score of color is founded upon prejudice, and hence cannot be removed by argument, for prejudice is blind and listens not to reason." -- Rep. Godlove S. Orth of Indiana, speech before the House of Representatives (5 Apr 1869) on the question of admitting the Dominican Republic as a US territory.
• "This persuasion of the power of the priest is, as we have said, a traditional prejudice; it is not founded on any reasons or proofs addressed to the understanding, and therefore it cannot be removed by argument." -- John Eliot Howard, The Island of the Saints (1855), quoting from the Achill Herald (Jun 1855).