Source: The Inefficient Stock Market - What Pays Off And Why (1999), Chapter 15, The Wrong 20-yard Line, p. 142
“Men are always more inclined to pitch their estimate of the enemy's strength too high than too low, such is human nature.”
On War (1832), Book 1
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Carl von Clausewitz 68
German-Prussian soldier and military theorist 1780–1831Related quotes

“3769. One may as much miss the Mark, by aiming too high, as too low.”
Introductio ad prudentiam: Part II (1727), Gnomologia (1732)

“My fees are not too high. Your wage scale may simply be too low.”
Source: Showboat World (1975), Chapter 12 (p. 132)

“Faulkner, more than most men, was aware of human strength as well as of human weakness.”
Nobel Prize acceptance speech (1962)
Context: Humanity has been passing through a gray and desolate time of confusion. My great predecessor, William Faulkner, speaking here, referred to it as a tragedy of universal fear so long sustained that there were no longer problems of the spirit, so that only the human heart in conflict with itself seemed worth writing about.
Faulkner, more than most men, was aware of human strength as well as of human weakness. He knew that the understanding and the resolution of fear are a large part of the writer's reason for being.
This is not new. The ancient commission of the writer has not changed. He is charged with exposing our many grievous faults and failures, with dredging up to the light our dark and dangerous dreams for the purpose of improvement.
On Alexander the Great, p. 312
Source: The Persian Boy (1972)
Context: It is better to believe in men too rashly, and regret, than believe too meanly. Men could be more than they are, if they would try for it. He has shown them that. How many have tried, because of him? Not only those I have seen; there will be men to come. Those who look in mankind only for their own littleness, and make them believe in that, kill more than he ever will in all his wars.

Attributed without citation in Ken Robinson, The Element (2009), p. 260. Widely attributed to Michelangelo since the late 1990s, this adage has not been found before 1980 when it appeared without attribution in E. C. McKenzie, Mac's giant book of quips & quotes.
Disputed
Variant: The greatest danger for most of us is not that our aim is too high and we miss it, but that it is too low and we reach it.

1962, Address and Question and Answer Period at the Economic Club of New York (549)