„I can quite understand why we cannot speak about the content of religion in an objectifying language. The fact that different religions try to express this content in quite distinct spiritual forms is no real objection. Perhaps we ought to look upon these different forms as complementary descriptions which, though they exclude one another, are needed to convey the rich possibilities flowing from man's relationship with the central order.“

—  Niels Bohr

Remarks after the Solvay Conference (1927)
Contesto: I consider those developments in physics during the last decades which have shown how problematical such concepts as "objective" and "subjective" are, a great liberation of thought. The whole thing started with the theory of relativity. In the past, the statement that two events are simultaneous was considered an objective assertion, one that could be communicated quite simply and that was open to verification by any observer. Today we know that 'simultaneity' contains a subjective element, inasmuch as two events that appear simultaneous to an observer at rest are not necessarily simultaneous to an observer in motion. However, the relativistic description is also objective inasmuch as every observer can deduce by calculation what the other observer will perceive or has perceived. For all that, we have come a long way from the classical ideal of objective descriptions.
In quantum mechanics the departure from this ideal has been even more radical. We can still use the objectifying language of classical physics to make statements about observable facts. For instance, we can say that a photographic plate has been blackened, or that cloud droplets have formed. But we can say nothing about the atoms themselves. And what predictions we base on such findings depend on the way we pose our experimental question, and here the observer has freedom of choice. Naturally, it still makes no difference whether the observer is a man, an animal, or a piece of apparatus, but it is no longer possible to make predictions without reference to the observer or the means of observation. To that extent, every physical process may be said to have objective and subjective features. The objective world of nineteenth-century science was, as we know today, an ideal, limiting case, but not the whole reality. Admittedly, even in our future encounters with reality we shall have to distinguish between the objective and the subjective side, to make a division between the two. But the location of the separation may depend on the way things are looked at; to a certain extent it can be chosen at will. Hence I can quite understand why we cannot speak about the content of religion in an objectifying language. The fact that different religions try to express this content in quite distinct spiritual forms is no real objection. Perhaps we ought to look upon these different forms as complementary descriptions which, though they exclude one another, are needed to convey the rich possibilities flowing from man's relationship with the central order.

„We depend on our words… Our task is to communicate experience and ideas to others.“

—  Niels Bohr

Quoted in Philosophy of Science Vol. 37 (1934), p. 157, and in The Truth of Science : Physical Theories and Reality (1997) by Roger Gerhard Newton, p. 176
Contesto: What is it that we humans depend on? We depend on our words... Our task is to communicate experience and ideas to others. We must strive continually to extend the scope of our description, but in such a way that our messages do not thereby lose their objective or unambiguous character … We are suspended in language in such a way that we cannot say what is up and what is down. The word "reality" is also a word, a word which we must learn to use correctly.

„In mathematics we can take our inner distance from the content of our statements. In the final analysis mathematics is a mental game that we can play or not play as we choose. Religion, on the other hand, deals with ourselves, with our life and death; its promises are meant to govern our actions and thus, at least indirectly, our very existence. We cannot just look at them impassively from the outside. Moreover, our attitude to religious questions cannot be separated from our attitude to society.“

—  Niels Bohr

Remarks after the Solvay Conference (1927)
Contesto: In mathematics we can take our inner distance from the content of our statements. In the final analysis mathematics is a mental game that we can play or not play as we choose. Religion, on the other hand, deals with ourselves, with our life and death; its promises are meant to govern our actions and thus, at least indirectly, our very existence. We cannot just look at them impassively from the outside. Moreover, our attitude to religious questions cannot be separated from our attitude to society. Even if religion arose as the spiritual structure of a particular human society, it is arguable whether it has remained the strongest social molding force through history, or whether society, once formed, develops new spiritual structures and adapts them to its particular level of knowledge. Nowadays, the individual seems to be able to choose the spiritual framework of his thoughts and actions quite freely, and this freedom reflects the fact that the boundaries between the various cultures and societies are beginning to become more fluid. But even when an individual tries to attain the greatest possible degree of independence, he will still be swayed by the existing spiritual structures — consciously or unconsciously. For he, too, must be able to speak of life and death and the human condition to other members of the society in which he's chosen to live; he must educate his children according to the norms of that society, fit into its life. Epistemological sophistries cannot possibly help him attain these ends. Here, too, the relationship between critical thought about the spiritual content of a given religion and action based on the deliberate acceptance of that content is complementary. And such acceptance, if consciously arrived at, fills the individual with strength of purpose, helps him to overcome doubts and, if he has to suffer, provides him with the kind of solace that only a sense of being sheltered under an all-embracing roof can grant. In that sense, religion helps to make social life more harmonious; its most important task is to remind us, in the language of pictures and parables, of the wider framework within which our life is set.

„In that sense, religion helps to make social life more harmonious; its most important task is to remind us, in the language of pictures and parables, of the wider framework within which our life is set.“

—  Niels Bohr

Remarks after the Solvay Conference (1927)
Contesto: In mathematics we can take our inner distance from the content of our statements. In the final analysis mathematics is a mental game that we can play or not play as we choose. Religion, on the other hand, deals with ourselves, with our life and death; its promises are meant to govern our actions and thus, at least indirectly, our very existence. We cannot just look at them impassively from the outside. Moreover, our attitude to religious questions cannot be separated from our attitude to society. Even if religion arose as the spiritual structure of a particular human society, it is arguable whether it has remained the strongest social molding force through history, or whether society, once formed, develops new spiritual structures and adapts them to its particular level of knowledge. Nowadays, the individual seems to be able to choose the spiritual framework of his thoughts and actions quite freely, and this freedom reflects the fact that the boundaries between the various cultures and societies are beginning to become more fluid. But even when an individual tries to attain the greatest possible degree of independence, he will still be swayed by the existing spiritual structures — consciously or unconsciously. For he, too, must be able to speak of life and death and the human condition to other members of the society in which he's chosen to live; he must educate his children according to the norms of that society, fit into its life. Epistemological sophistries cannot possibly help him attain these ends. Here, too, the relationship between critical thought about the spiritual content of a given religion and action based on the deliberate acceptance of that content is complementary. And such acceptance, if consciously arrived at, fills the individual with strength of purpose, helps him to overcome doubts and, if he has to suffer, provides him with the kind of solace that only a sense of being sheltered under an all-embracing roof can grant. In that sense, religion helps to make social life more harmonious; its most important task is to remind us, in the language of pictures and parables, of the wider framework within which our life is set.

„Truth and clarity are complementary.“

—  Niels Bohr

As quoted in Quantum Theory and the Flight from Realism : Philosophical Responses to Quantum Mechanics (2000) by Christopher Norris, p. 234
Originale: (ro) Contraria Sunt Complementa
Variante: Opposites are complementary.

„No, no, you are not thinking, you are just being logical.“

—  Niels Bohr

In response to those who made purely formal or mathematical arguments, as quoted in What Little I Remember (1979) by Otto Robert Frisch, p. 95

„Prediction is very difficult, especially about the future.“

—  Niels Bohr

As quoted in Teaching and Learning Elementary Social Studies (1970) by Arthur K. Ellis, p. 431
The above quote is also attributed to various humourists and the Danish poet Piet Hein: "det er svært at spå – især om fremtiden"
It is also attributed to Danish cartoonist Storm P (Robert Storm Petersen).
Disputed
Variante: It's hard to make predictions, especially about the future.

„It is not enough to be wrong, one must also be polite.“

—  Niels Bohr

As quoted in The Genius of Science: A Portrait Gallery (2000) by Abraham Pais, p. 24

„Every valuable human being must be a radical and a rebel, for what he must aim at is to make things better than they are.“

—  Niels Bohr

As quoted in The World of the Atom (1966) by Henry Abraham Boorse and Lloyd Motz, p. 741

„It is a great pity that human beings cannot find all of their satisfaction in scientific contemplativeness.“

—  Niels Bohr

As quoted in Chandra: A Biography of S. Chandrasekhar‎ (1991) by Kameshwar C. Wali, p. 147

„Anyone who is not shocked by quantum theory has not understood it.“

—  Niels Bohr

Bohr said this sentence in a conversation with Werner Heisenberg, as quoted in: "Der Teil und das Ganze. Gespräche im Umkreis der Atomphysik" . R. Piper & Co., München, 1969, S. 280. DIE ZEIT 22. Aug. 1969 http://www.zeit.de/1969/34/kein-chaos-aus-dem-nicht-wieder-ordnung-wuerde.
As quoted in Meeting the Universe Halfway (2007) by Karen Michelle Barad, p. 254 http://books.google.com/books?id=4qYorOpfB6EC&lpg=PP1&pg=PA254#v=onepage&q&f=false, with the quote attributed to The Philosophical Writings of Niels Bohr, but with no page number or volume number given.
David Mermin, on pages 186– 187 http://books.google.com/books?id=bf5bjBk095UC&lpg=PP1&pg=PA187#v=onepage&q&f=false of his book Boojums All the Way Through: Communicating Science in a Prosaic Age (1990) noted that he specifically looked for pithy quotes about quantum mechanics along these lines when reviewing the three volumes of The Philosophical Writings of Niels Bohr, but couldn't find any: <blockquote>Once I tried to teach some quantum mechanics to a class of law students, philosophers, and art historians. As an advertisement for the course I put together the most sensational quotations I could collect from the most authoritative practitioners of the subject. Heisenberg was a goldmine: “The concept of the objective reality of the elementary particles has thus evaporated…”; “the idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist, independently of whether or not we observe them … is impossible …” Feynman did his part too: “I think I can safely say that nobody understands quantum mechanics.” But I failed to turn up anything comparable in the writings of Bohr. Others attributed spectacular remarks to him, but he seemed to take pains to avoid any hint of the dramatic in his own writings. You don't pack them into your classroom with “The indivisibility of quantum phenomena finds its consequent expression in the circumstance that every definable subdivision would require a change of the experimental arrangement with the appearance of new individual phenomena,” or “the wider frame of complementarity directly expresses our position as regards the account of fundamental properties of matter presupposed in classical physical description but outside its scope.”<p>I was therefore on the lookout for nuggets when I sat down to review these three volumes – a reissue of Bohr's collected essays on the revolutionary epistemological character of the quantum theory and on the implications of that revolution for other scientific and non-scientific areas of endeavor (the originals first appeared in 1934, 1958, and 1963.) But the most radical statement I could find in all three books was this: "...physics is to be regarded not so much as the study of something a priori given, but rather as the development of methods for ordering and surveying human experience." No nuggets for the nonscientist.</blockquote>
Variants: Those who are not shocked when they first come across quantum mechanics cannot possibly have understood it.
Those who are not shocked when they first come across quantum theory cannot possibly have understood it.
Anyone who is not shocked by quantum theory has not understood a single word.
If you think you can talk about quantum theory without feeling dizzy, you haven't understood the first thing about it.

„Never express yourself more clearly than you are able to think.“

—  Niels Bohr

As quoted in Values of the Wise : Humanity's Highest Aspirations (2004) by Jason Merchey, p. 63

„How wonderful that we have met with a paradox. Now we have some hope of making progress.“

—  Niels Bohr

As quoted in Niels Bohr : The Man, His Science, & the World They Changed (1966) by Ruth Moore, p. 196

„Of course not … but I am told it works even if you don't believe in it.“

—  Niels Bohr

Reply to a visitor to his home in Tisvilde who asked him if he really believed a horseshoe above his door brought him luck, as quoted in Inward Bound : Of Matter and Forces in the Physical World (1986) by Abraham Pais, p. 210
In most published accounts of this anecdote such was Bohr's reply to his friend, but in one early account, in The Interaction Between Science and Philosophy (1974) by Samuel Sambursky, p. 357, Bohr was at a friend's house and asked "Do you really believe in this?" to which his friend replied "Oh, I don't believe in it. But I am told it works even if you don't believe in it."
Disputed
Variante: No, but I'm told it works even if you don't believe in it.

„Every sentence I utter must be understood not as an affirmation, but as a question.“

—  Niels Bohr

As quoted in A Dictionary of Scientific Quotations (1991) by Alan L. Mackay, p. 35

„Some subjects are so serious that one can only joke about them.“

—  Niels Bohr

As quoted in The Genius of Science: A Portrait Gallery (2000) by Abraham Pais, p. 24
Some things are so serious that one can only joke about them.
Variant without any citation as to author in Denial is not a river in Egypt (1998) by Sandi Bachom, p. 85.

„I go into the Upanishads to ask questions.“

—  Niels Bohr

As quoted in God Is Not One : The Eight Rival Religions That Run the World and Why Their Differences Matter (2010), by Stephen Prothero, Ch, 4 : Hinduism : The Way of Devotion, p. 144

„We are all agreed that your theory is crazy. The question that divides us is whether it is crazy enough to have a chance of being correct.“

—  Niels Bohr

Said to Wolfgang Pauli after his presentation of Heisenberg's and Pauli's nonlinear field theory of elementary particles, at Columbia University (1958), as reported by F. J. Dyson in his paper “Innovation in Physics” (Scientific American, 199, No. 3, September 1958, pp. 74-82; reprinted in "JingShin Theoretical Physics Symposium in Honor of Professor Ta-You Wu," edited by Jong-Ping Hsu & Leonardo Hsu, Singapore; River Edge, NJ: World Scientific, 1998, pp. 73-90, here: p. 84).
Your theory is crazy, but it's not crazy enough to be true.
As quoted in First Philosophy: The Theory of Everything (2007) by Spencer Scoular, p. 89
There are many slight variants on this remark:
We are all agreed that your theory is crazy. The question which divides us is whether it is crazy enough.
We are all agreed that your theory is crazy. The question is whether it is crazy enough to be have a chance of being correct.
We in the back are convinced your theory is crazy. But what divides us is whether it is crazy enough.
Your theory is crazy, the question is whether it's crazy enough to be true.
Yes, I think that your theory is crazy. Sadly, it's not crazy enough to be believed.

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