“We must nevertheless present all possible interpretations for each observation, so that competing theories can be formulated and defended. In science, as elsewhere, intellectual inertia, the fashions of the moment, the weight of institutions, and authoritarianism are always to be feared. Heresies play an essential role by keeping our minds argumentative and alert.”

The Evolution of Physics (1938) (co-written with Leopold Infeld)
1930s

"The Nearest Star" (1989) (reprinted in The Secret of the Universe (1992), p. 82)
General sources

Source: 1940s-1950s, Administrative Behavior, 1947, p. 43.

Source: Silence Speaks, from the chalkboard of Baba Hari Dass, 1977, p.11

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conjecture
in his Nobel Lecture http://nobelprize.org/nobel_prizes/chemistry/laureates/1996/curl-lecture.html, December 7, 1996, Dawn of the Fullerenes: Experiment and Conjecture

1910s, Nobel lecture (1910)
Context: In new and wild communities where there is violence, an honest man must protect himself; and until other means of securing his safety are devised, it is both foolish and wicked to persuade him to surrender his arms while the men who are dangerous to the community retain theirs. He should not renounce the right to protect himself by his own efforts until the community is so organized that it can effectively relieve the individual of the duty of putting down violence. So it is with nations. Each nation must keep well prepared to defend itself until the establishment of some form of international police power, competent and willing to prevent violence as between nations. As things are now, such power to command peace throughout the world could best be assured by some combination between those great nations which sincerely desire peace and have no thought themselves of committing aggressions. The combination might at first be only to secure peace within certain definite limits and on certain definite conditions; but the ruler or statesman who should bring about such a combination would have earned his place in history for all time and his title to the gratitude of all mankind.

"Matter" in Man's Right to Knowledge, 2nd series (1954), p. 10; also in Writings on Physics and Philosophy‎ (1994) edited by Charles Paul P. Enz and Karl von Meyenn
Context: In the new pattern of thought we do not assume any longer the detached observer, occurring in the idealizations of this classical type of theory, but an observer who by his indeterminable effects creates a new situation, theoretically described as a new state of the observed system. In this way every observation is a singling out of a particular factual result, here and now, from the theoretical possibilities, therefore making obvious the discontinuous aspect of physical phenomena.
Nevertheless, there remains still in the new kind of theory an objective reality, inasmuch as these theories deny any possibility for the observer to influence the result of a measurement, once the experimental arrangement is chosen. Therefore particular qualities of an individual observer do not enter into the conceptual framework of the theory.