“Never attribute to malevolence what is merely due to incompetence”

Prologue. p. xv.
Understanding Uncertainty (2006)

Source: Space Opera (1965), Chapter 4 (p. 30)

Variant: I have no idea what I am doing but incompetence has never prevented me from plunging in with enthusiasm.

Mullane v. Central Hanover Bank & Trust Co., 339 U.S. 306 (1950)
Judicial opinions

Source: The Scarlet Letter

Source: The Peter Principle (1969), p. 107 (The Mathematics of Incompetence)

Source: Science is Not Enough (1967), p. 28 - 29
Context: We puzzle as to whether the universe is bounded or extends forever; whether, indeed, it may only be one universe among many. We speculate as to whether our universe began in a vast explosion, whether it pulsates between utter compression and wide diffusion, whether it is self-renewing and thus unchanged forever. And we are humble.
But science teaches more than this. It continually reminds us that we are still ignorant and there is much to learn. Time and space are interconnected in strange ways; there is no absolute simultaneity. Within the atom occur phenomena concerning which visualization is futile, to which common sense, the guidance from our everyday experience, has no application, which yield to studies by equations that have no meaning except that they work. Mass and energy transform one into another, Gravitation, the solid rock on which Newton built, may be merely a property of the geometry of the cosmos. Life, as its details unfold before us, becomes ever more intricate, emphasizing more and more our wonder that its marvelous functioning could have been produced by chance and time. The human mind, merely in its chemical and physical aspects, takes on new inspiring attributes.
And what is the conclusion? He who follows science blindly, and who follows it alone, comes to a barrier beyond which he cannot see. He who would tell us with the authority of scholarship a complete story of why we exist, of our mission here, has a duty to speak convincingly in a world where men increasingly think for themselves. Exhortation needs to be revised, not to weaken its power, but to increase it, for men who are no longer in the third century. As this occurs, and on the essential and central core of faith, science will of necessity be silent.
But its silence will be the silence of humility, not the silence of disdain. A belief may be larger than a fact. A faith that is overdefined is the very faith most likely to prove inadequate to the great moments of life.

Source: My Forty Years with Ford, 1956, p. 51; Quoted in The Rotarian, Oct. 1976, p. 56