“It is sheer nonsense to expect that any human being has yet been able to attain such insight into the problems of society that he can really identify the central problems and determine how they should be solved. The systems in which we live are far too complicated as yet for our intellectual powers and technology to understand.”

1930s- 1950s, The New Society (1950)

Smuts on the rebuilding of South Africa after the Boer War, in The Theory of Holism, 1940, p. 133

"Simpler Social Problems Have Proved Intractable", paragraph 138
Industrial Society and Its Future (1995)

preface to 2015 edition of Secrets and Lies
Cryptography
Context: A few years ago I heard a quotation, and I am going to modify it here: If you think technology can solve your security problems, then you don't understand the problems and you don't understand the technology.

Section 9 : Ethical Outlook
Life and Destiny (1913)

1963, American University speech
Context: Let us examine our attitude toward peace itself. Too many of us think it is impossible. Too many think it unreal. But that is a dangerous, defeatist belief. It leads to the conclusion that war is inevitable — that mankind is doomed — that we are gripped by forces we cannot control. We need not accept that view. Our problems are manmade — therefore, they can be solved by man. And man can be as big as he wants. No problem of human destiny is beyond human beings. Man's reason and spirit have often solved the seemingly unsolvable — and we believe they can do it again.

Source: Brave New World Revisited (1958), Chapter 3 (p. 19)

letter to Koichi Mano (3 February 1966); published in Perfectly Reasonable Deviations from the Beaten Track: The Letters of Richard P. Feynman (2005), p. 198, 201
also quoted by Freeman Dyson in "Wise Man" http://www.nybooks.com/articles/18350, The New York Review of Books (20 October 2005)
Context: The worthwhile problems are the ones you can really solve or help solve, the ones you can really contribute something to. … No problem is too small or too trivial if we can really do something about it. You say you are a nameless man. You are not to your wife and to your child. You will not long remain so to your immediate colleagues if you can answer their simple questions when they come into your office. You are not nameless to me. Do not remain nameless to yourself — it is too sad a way to be. Know your place in the world and evaluate yourself fairly, not in terms of the naïve ideals of your own youth, nor in terms of what you erroneously imagine your teacher's ideals are.

Source: Christianity and the Social Crisis (1907), Introduction, p. xi