“The question is have we met our match?”

1960, The New Frontier
Context: For the harsh facts of the matter are that we stand on this frontier at a turning-point in history. We must prove all over again whether this nation — or any nation so conceived — can long endure — whether our society — with its freedom of choice, its breadth of opportunity, its range of alternatives — can compete with the single-minded advance of the Communist system. Can a nation organized and governed such as ours endure? That is the real question. Have we the nerve and the will? Can we carry through in an age where we will witness not only new breakthroughs in weapons of destruction — but also a race for mastery of the sky and the rain, the ocean and the tides, the far side of space and the inside of men's minds? Are we up to the task — are we equal to the challenge? Are we willing to match the Russian sacrifice of the present for the future — or must we sacrifice our future in order to enjoy the present? That is the question of the New Frontier. That is the choice our nation must make — a choice that lies not merely between two men or two parties, but between the public interest and private comfort — between national greatness and national decline — between the fresh air of progress and the stale, dank atmosphere of "normalcy" — between determined dedication and creeping mediocrity. All mankind waits upon our decision. A whole world looks to see what we will do. We cannot fail their trust, we cannot fail to try.

Source: Wise Children

Quoted in Friedman, Robert I., Zealots for Zion: Inside Israel's West Bank Settlement Movement (New York, New York: Random House, 1992), 132-52.
1990s

Source: Why We Love Dogs, Eat Pigs, and Wear Cows (2010), p. 18

Source: How To Write A Sentence And How To Read One (2011), Chapter 1, Why Sentences?, p. 4

This has also appeared in the alternate form: "What we observe is not nature itself, but nature exposed to our method of questioning."
Physics and Philosophy (1958)
Variant: What we observe is not nature itself, but nature exposed to our method of questioning.
Source: Physics and Philosophy: The Revolution in Modern Science

Without Me, You're Nothing: The Essential Guide to Home Computers (1981), co-written with Max Barnard
General sources

We don't need to give away flags for our fans to wave (2012)