Sourced

• We must be clear that when it comes to atoms, language can be used only as in poetry. The poet, too, is not nearly so concerned with describing facts as with creating images and establishing mental connections.
  • In his first meeting with Werner Heisenberg in early summer 1920, in response to questions on the nature of language, as reported in Discussions about Language (1933); quoted in Defense Implications of International Indeterminacy (1972) by Robert J. Pranger, p. 11, and Theorizing Modernism : Essays in Critical Theory (1993) by Steve Giles, p. 28

• The great extension of our experience in recent years has brought light to the insufficiency of our simple mechanical conceptions and, as a consequence, has shaken the foundation on which the customary interpretation of observation was based.
  • "Atomic Physics and the Description of Nature" (1934)

• Isolated material particles are abstractions, their properties being definable and observable only through their interaction with other systems.
  • "Atomic Physics and the Description of Nature" (1934)

• 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.
  • 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

• For a parallel to the lesson of atomic theory regarding the limited applicability of such customary idealisations, we must in fact turn to quite other branches of science, such as psychology, or even to that kind of epistemological problems with which already thinkers like Buddha and Lao Tzu have been confronted, when trying to harmonize our position as spectators and actors in the great drama of existence.
  • Speech on quantum theory at Celebrazione del Secondo Centenario della Nascita di Luigi Galvani, Bologna, Italy (October 1937)

• 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.
  • Said to Wolfgang Pauli after his presentation of Heisenberg's and Pauli's nonlinear field theory of elementary particles, at Columbia University (1958), as quoted in Symposium on Basic Research (1959) by Dael Lee Wolfle, p. 66
  • 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 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.

• Physics is to be regarded not so much as the study of something a priori given, but rather as the development of methods of ordering and surveying human experience. In this respect our task must be to account for such experience in a manner independent of individual subjective judgement and therefor objective in the sense that it can be unambiguously communicated in ordinary human language.
  • "The Unity of Human Knowledge" (October 1960)

• There is no quantum world. There is only an abstract physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature...
  • As quoted in "The philosophy of Niels Bohr" by Aage Petersen, in the Bulletin of the Atomic Scientists Vol. 19, No. 7 (September 1963); The Genius of Science: A Portrait Gallery (2000) by Abraham Pais, p. 24, and Niels Bohr: Reflections on Subject and Object (2001) by Paul. McEvoy, p. 291

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

• How wonderful that we have met with a paradox. Now we have some hope of making progress.
  • As quoted in Niels Bohr : The Man, His Science, & the World They Changed (1966) by Ruth Moore, p. 196

• Two sorts of truth: trivialities, where opposites are obviously absurd, and profound truths, recognised by the fact that the opposite is also a profound truth.
  • As quoted by his son Hans Bohr in "My Father", published in Neils Bohr : His Life and Work (1967), p. 328
  • Unsourced variant : The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth.
  • As quoted in Max Delbrück, Mind from Matter: An Essay on Evolutionary Epistemology, (1986) p. 167. It is the hallmark of any deep truth that its negation is also a deep truth

• An expert is a man who has made all the mistakes which can be made in a very narrow field.
  • As quoted by Edward Teller (10 October 1972), and A Dictionary of Scientific Quotations (1991) by Alan L. Mackay, p. 35

• Every sentence I utter must be understood not as an affirmation, but as a question.
  • As quoted in A Dictionary of Scientific Quotations (1991) by Alan L. Mackay, p. 35

• Anyone who is not shocked by quantum theory has not understood it.
  • As quoted in Meeting the Universe Halfway (2007) by Karen Michelle Barad, p. 254, with a footnote citing The Philosophical Writings of Niels Bohr (1998).
  • 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.

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

• Truth and clarity are complementary.
  • As quoted in Quantum Theory and the Flight from Realism : Philosophical Responses to Quantum Mechanics (2000) by Christopher Norris, p. 234

• It is not enough to be wrong, one must also be polite.
  • As quoted in The Genius of Science: A Portrait Gallery (2000) by Abraham Pais, p. 24

• Never express yourself more clearly than you are able to think.
  • As quoted in Values of the Wise : Humanity's Highest Aspirations (2004) by Jason Merchey, p. 63

Disputed

• It is better to not understand something true, than to understand something false.

• Prediction is very difficult, especially about the future.
  • 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).

• Stop telling God what to do with his dice.
  • A response to Einstein's assertion that "God doesn't play dice"; a similar statement is attributed to Enrico Fermi
  • Variant: Einstein, don't tell God what to do.

• Of course not ... but I am told it works even if you don't believe in it.
  • 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 the earliest account thus far located, 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."
  • Variant: No, but I'm told it works even if you don't believe in it.

Unsourced

• I go into the Upanishads to ask questions.

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

Quotes about Bohr

• Not often in life has a human being caused me such joy by his mere presence as you did.
  • Albert Einstein in a letter to Bohr (1920)

• I remember discussions with Bohr which went through many hours till very late at night an ended almost in despair; and when at the end of the discussion I went alone for a walk in the neighbouring park I repeated to myself again and again the question: Can nature possibly be so absurd as it seemed to us in these atomic experiments?
  • Werner Heisenberg in Physics and Philosophy (1958)

• One of the favorite maxims of my father was the distinction between the two sorts of truths, profound truths recognized by the fact that the opposite is also a profound truth, in contrast to trivialities where opposites are obviously absurd.
  • Hans Henrik Bohr, writing about his father in "My father" in Niels Bohr - His Life and Work As Seen By His Friends and Colleagues (1967), S. Rozental, ed.