Known sourcecanonical
"Though man a thinking being is defined, Few use the grand prerogative of mind. How few think justly of the thinking few! How many never think, who think they do!"
Jane Taylor / Essays in Rhyme. (On Morals and Manners. Prejudice.) Essay i. Stanza 45.
Essays in Rhyme. (On Morals and Manners. Prejudice.) Essay i. Stanza 45.
Bartlett's Familiar Quotations, 9th ed. (Little, Brown, 1905), public domain