Though sometimes credited to Ziglar on the internet, this is credited to Marie Fraser in Quote Unquote (1977) by Lloyd Cory
Misattributed
“Of all the discoveries which men need to make, the most important, at the present moment, is that of the self-forming power treasured up in themselves. They little suspect its extent, as little as the savage apprehends the energy which the mind is created to exert on the material world.”
Self-Culture (1838)
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William Ellery Channing 71
United States Unitarian clergyman 1780–1842Related quotes

Source: 1910s, Mysticism and Logic and Other Essays http://archive.org/stream/mysticism00russuoft/mysticism00russuoft_djvu.txt (1918), Ch. 1: Mysticism and Logic

Howard Zinn on War (2000), Ch. 14: Vietnam: A Matter of Perspective http://www.thirdworldtraveler.com/Zinn/Vietnam_Perspective_HZOW.html
Context: Scholars, who pride themselves on speaking their minds, often engage in a form of self-censorship which is called "realism." To be "realistic" in dealing with a problem is to work only among the alternatives which the most powerful in society put forth. It is as if we are all confined to a, b, c, or d in the multiple choice test, when we know there is another possible answer. American society, although it has more freedom of expression than most societies in the world, thus sets limits beyond which respectable people are not supposed to think or speak. So far, too much of the debate on Vietnam has observed these limits.

Letter to Nikita Khrushchev after JFK assassination, as quoted in One Minute to Midnight: Kennedy, Khrushchev, and Castro on the Brink of Nuclear War (2009) by Michael Dobbs.

Source: Défense des Lettres [In Defense of Letters] (1937), p. xii

pp. 70–71 https://archive.org/stream/ActivationOfEnergy/Activation_of_Energy#page/n65/mode/2up
Activation of Energy (1976)

Loud and continued cheers.
Speech in Birmingham (15 May 1903), quoted in The Times (16 May 1903), p. 8
1900s