Turns out it's a bit paraphrased.
"A statesman who aims at perfection, and thinks that he knows how to reach it, will stop at nothing to drive others along the same road, and his political ideals will be inextricably mixed up with his desire to remain in power. Perfection, in practice, is never attained, and the terrorism used in pursuit of it simply breeds the need for fresh terrorism. Consequently the attempt to establish liberty and equality always ends in the police state: whereas more limited aims, based on the realisation that man's nature is full of evil, may lead to a fairly decent society."