Eric Hoffer a.k.a. “Longshoreman Philosopher” (July 25, 1902 – May 21, 1983)
American Social Writer, Author, Awarded the Presidential Medal of Freedom
The blindness of the fanatic is a source of strength (he sees no obstacles), but it is the cause of intellectual sterility and emotional monotony. The fanatic is also mentally cocky, and hence barren of new beginnings. At the root of his cockiness is the conviction that life and the universe conform to a simple formula – his formula. His is thus without the fruitful intervals of groping [uncertainty], when the mind is as it were in solution – ready for all manner of new reactions, new combinations and new beginnings. (Between the Devil and the Dragon. New York: Harper & Row Publishers, 1982, p. 287)
Always Question Authority 
Thanks blogmeister for reminding me of one of the great minds and writers of his and any time. My political formation was greatly influenced by Hoffer’s True Believer when I first read him in the 1960s. His great strength is the accessibility of his material. Check out his The Passionate State of Mind as well.