The truth seems to be that propaganda on its own cannot force its way into unwilling minds; neither can it inculcate something wholly new; nor can it keep people persuaded once they have ceased to believe. It penetrates only into minds already open, and rather than instill opinion it articulates and justifies opinions already present in the minds of its recipients. The gifted propagandist brings to a boil ideas and passions already simmering in the minds of his hearers. he echoes their innermost feelings. Where opinion is not coerced, people can be made to believe only in what they already "know."

The True Believer (1951) - Part Three: United Action and Self-Sacrifice