"Nothing exists except atoms and empty space; everything else is opinion." [ca. 400 BC]
"If I have seen farther than others, it has been by standing on the shoulders of giants."
"Hypotheses non fingo." [I do not engage in idle speculation.]
"That one body may act upon another at a distance through a vacuum without a mediation of anything else by or through which their action of force may be conveyed from one to another is to me so great an absurdity that I believe that no man who has in philosophical matters any competent faculty of thinking can ever fall into it." 
"To the electron: may it never be of any use!" [said to have been posted on the Cavendish Lab wall]
"In science there is only Physics; all the rest is stamp collecting."
(1912) "Bohr's different. He's a football player!" [alleged comment on Continental theorists]
"I want to know God's thoughts; the rest is just details."
"What really interests me is whether God had any choice in the creation of the world."
"The most incomprehensible thing about the Universe is that it is comprehensible."
(after Heisenberg's 1927 lecture) "Marvelous, what ideas the young people have these days. But I don't believe a word of it."
"Gott wurfelt nicht!" [God does not play dice!]
"Quantum mechanics is very impressive. But an inner voice tells me that it is not yet the real thing. The theory produces a good deal but hardly brings us closer to the secret of the Old One. I am at all events convinced that He does not play dice."
"It seems hard to look in God's cards. But I cannot for a moment believe that He plays dice and makes use of `telepathic' means (as the current quantum theory alleges He does)."
"What nature demands from us is not a quantum theory or a wave theory; rather, nature demands from us a synthesis of these two views which thus far has exceeded the mental powers of physicists."
"However I cannot seriously believe in it because the theory is incompatible with the principle that physics is to represent reality in space and time, without spookish long-distance effects."
"The Heisenberg-Bohr tranquilizing philosophy - or religion? - is so delicately contrived that, for the time being, it provides a gentle pillow for the true believer from which he cannot very easily be aroused. So let him lie there." [Einstein and Bohr did not often see eye to eye.] - contributed by Jeff La Porte
"But in physics I soon learned to scent out the paths that led to the depths, and to disregard everything else, all the many things that clutter up the mind, and divert it from the essential. The hitch in this was, of course, the fact that one had to cram all this stuff into one's mind for the examination, whether one liked it or not."
"For an academic career puts a young man into a kind of embarrassing position by requiring him to produce scientific publications in impressive quantity - a seduction into superficiality which only strong characters are able to withstand. Most practical occupations, however, are of such a nature that a man of normal ability is able to accomplish what is expected of him. His day-to-day existence does not depend on any special illuminations. If he has deeper scientific interests he may plunge into his favorite problems in addition to doing his required work. He need not be oppressed by the fear that his efforts may lead to no results. I owed it to Marcel Grossman that I was in such a fortunate position."
"I have become an obstinate heretic in the eyes of my colleagues."
"I am generally regarded as a sort of petrified object, rendered blind and deaf by the years. I find this role not too distasteful, as it corresponds very well with my temperament."
"Success carries more power of conviction for most people than reflections on principle."
"A quaint ceremonious village of puny demigods on stilts." [first impression of Princeton]
"Anyone who is not shocked by quantum theory has not understood a single word."
"The very nature of the quantum theory ... forces us to regard the space-time coordination and the claim of causality, the union of which characterizes the classical theories, as complementary but exclusive features of the description, symbolizing the idealization of observation and description, respectively."
"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."
"Prediction is very difficult, especially about the future."
"An expert is a person who has made all the mistakes that can be made in a very narrow field."
"Never express yourself more clearly than you are able to think."
"Your theory is crazy, but it's not crazy enough to be true." [to a young Physicist]
"Very interesting theory - it makes no sense at all."
"I myself . . . only came to believe in the uncertainty relations after many pangs of conscience. . . . "
"Had I known that we were not going to get rid of this damned quantum jumping, I never would have involved myself in this business!"
"One does not get an answer to the question, `What is the state after collision?' but only to the question, `How probable is a given effect of the collision?' From the standpoint of our quantum mechanics, there is no quantity which causally fixes the effect of a collision in an individual event. Should we hope to discover such properties later ... and determine [them] in individual events? ... I myself am inclined to renounce determinism in the atomic world, but that is a philosophical question for which physical arguments alone do not set standards."
"Science is the belief in the ignorance of experts."
"I was born not knowing, and have only had a little time to change that here and there."
"A philosopher once said, `It is necessary for the very existence of science that the same conditions always produce the same results.' Well, they don't!" (1965)
(on quantum mechanics) "The paradox is only a conflict between reality and your feeling what reality ought to be."
"We have always had a great deal of difficulty understanding the world view that quantum mechanics represents. At least I do, because I'm an old enough man that I haven't got to the point that this stuff is obvious to me. Okay, I still get nervous with it.... You know how it always is, every new idea, it takes a generation or two until it becomes obvious that there's no real problem. I cannot define the real problem, therefore I suspect there's no real problem, but I'm not sure there's no real problem." (1982)
[with John Archibald Wheeler in Rev. Mod. Phys. 21, 425 (1949)]: "However unfamiliar this direct interparticle treatment compared to the electrodynamics of Maxwell and Lorentz, it deals with the same problems, talks about the same charges, considers the interactions of the same current elements, obtains the same capacitances, predicts the same inductances and yields the same physical conclusions. Consequently action-at-a-distance must have a close connection with field theory."
"I am always surprised when a young man tells me he want to work at cosmology; I think of cosmology as something that happens to one, not something one can choose."
`I can't believe that!' said Alice.
`Can't you?' the Queen said in a pitying tone. `Try again: draw a long breath, and shut your eyes.'
Alice laughed. `There's no use trying,' she said; `one cannot believe impossible things.'
`I daresay you haven't had much practice,' said the Queen. `When I was your age, I always did it for half-an-hour a day. Why, sometimes I've believed as many as six impossible things before breakfast.'
`The name of the song is called "Haddocks' Eyes."'
`Oh, that's the name of the song, is it?' Alice said, trying to feel interested.
`No, you don't understand,' the Knight said, looking a little vexed. `That's what the name is called. The name really is "The Aged Aged Man."'
`Then I ought to have said "That's what the song is called"?' Alice corrected herself.
`No, you oughtn't: that's quite another thing! The song is called "Ways and Means": but that's only what it's called, you know!'
`Well, what is the song, then?' said Alice, who was by this time completely bewildered.
`I was coming to that,' the Knight said. `The song really is "A-sitting On A Gate": and the tune's my own invention.'
[D. Bohm et al., Phys. Rep. 144, 321 (1987)]: "If the price of avoiding non-locality is to make an intuitive explanation impossible, one has to ask whether the cost is too great."
"He lives below the senseless stars and writes his meanings in them."