“To know how much there is that we do not know, is one of the most valuable parts of our attainments; for such knowledge becomes both a lesson of humility and a stimulus to exertion.”
Lecture 6
Lectures on Education (1855)
Context: The most ignorant are the most conceited. Unless a man knows that there is something more to be known, his inference is, of course, that he knows every thing. Such a man always usurps the throne of universal knowledge, and assumes the right of deciding all possible questions. We all know that a conceited dunce will decide questions extemporaneous which would puzzle a college of philosophers, or a bench of judges. Ignorant and shallow-minded men do not see far enough to see the difficulty. But let a man know that there are things to be known, of which he is ignorant, and it is so much carved out of his domain of universal knowledge. And for all purposes of individual character, as well as of social usefulness, it is quite as important for a man to know the extent of his own ignorance as it is to know any thing else. To know how much there is that we do not know, is one of the most valuable parts of our attainments; for such knowledge becomes both a lesson of humility and a stimulus to exertion.
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Horace Mann 67
American politician 1796–1859Related quotes

Source: Homily of Most. Rev. Honesto Ongtioco, DD at the Mass of the Holy Spirit https://lst.edu/articles/homily-of-most-rev-honesto-ongtioco-dd-at-the-mass-of-the-holy-spirit-held-on-august-26-2015/ (August 26, 2015)

“Till we know that, what is all our knowledge; how shall we even so much as "detect?”
For the vulpine sharpness, which considers itself to be knowledge, and "detects" in that fashion, is far mistaken. Dupes indeed are many: but, of all dupes, there is none so fatally situated as he who lives in undue terror of being duped.
1840s, Heroes and Hero-Worship (1840), The Hero As King

Recreation (1919)
Context: There is much poetry for which most of us do not care, but with a little trouble when we are young we may find one or two poets whose poetry, if we get to know it well, will mean very much to us and become part of ourselves... The love for such poetry which comes to us when we are young will not disappear as we get older; it will remain in us, becoming an intimate part of our own being, and will be an assured source of strength, consolation, and delight.

“Is Knowledge knowable? If not, how do we know?”
Source: The Insanity Defense: The Complete Prose

“We know it's possible. We know it's valuable. We should do it.”
Merkle's website http://www.merkle.com/papers/nanohearing1999.html
Context: Developing nanotechnology will be a major project -- just as developing nuclear weapons or lunar rockets were major projects. We must first focus our efforts on developing two things: the tools with which to build the first molecular machines, and the blueprints of what we are to build. This will require the cooperative efforts of researchers across a wide range of disciplines: scanning probe microscopy, supramolecular chemistry, protein engineering, self assembly, robotics, materials science, computational chemistry, self replicating systems, physics, computer science, and more. This work must focus on fundamentally new approaches and methods: incremental or evolutionary improvements will not be sufficient. Government funding is both appropriate and essential for several reasons: the benefits will be pervasive across companies and the economy; few if any companies will have the resources to pursue this alone; and development will take many years to a few decades (beyond the planning horizon of most private organizations).
We know it's possible. We know it's valuable. We should do it.

“This is the most valuable lesson one can teach a fanatic: that fanaticism is self-defeating.”
Vergere, p. 291
Traitor (2002)

As quoted in A Year with the Saints (1891) by Anonymous, p. 47