Richard Feynman: Doing

The Value of Science (1955)
Context: The scientist has a lot of experience with ignorance and doubt and uncertainty, and this experience is of very great importance, I think. When a scientist doesn’t know the answer to a problem, he is ignorant. When he has a hunch as to what the result is, he is uncertain. And when he is pretty darn sure of what the result is going to be, he is still in some doubt. We have found it of paramount importance that in order to progress we must recognize our ignorance and leave room for doubt. Scientific knowledge is a body of statements of varying degrees of certainty — some most unsure, some nearly sure, but none absolutely certain. Now, we scientists are used to this, and we take it for granted that it is perfectly consistent to be unsure, that it is possible to live and not know. But I don’t know whether everyone realizes this is true. Our freedom to doubt was born out of a struggle against authority in the early days of science. It was a very deep and strong struggle: permit us to question — to doubt — to not be sure. I think that it is important that we do not forget this struggle and thus perhaps lose what we have gained.

Source: QED: The Strange Theory of Light and Matter (1985), p. 17

lecture I: "The Uncertainty of Science"
The Meaning of It All (1999)

interview published in Superstrings: A Theory of Everything? (1988) edited by Paul C. W. Davies and Julian R. Brown, p. 208-209
Context: God was always invented to explain mystery. God is always invented to explain those things that you do not understand. Now, when you finally discover how something works, you get some laws which you're taking away from God; you don't need him anymore. But you need him for the other mysteries. So therefore you leave him to create the universe because we haven't figured that out yet; you need him for understanding those things which you don't believe the laws will explain, such as consciousness, or why you only live to a certain length of time — life and death — stuff like that. God is always associated with those things that you do not understand. Therefore I don't think that the laws can be considered to be like God because they have been figured out.

statement after an introduction mentioning that he played bongo drums; Messenger Lectures at Cornell University, p. 13
The Character of Physical Law (1965)

The Value of Science (1955)
Context: We are at the very beginning of time for the human race. It is not unreasonable that we grapple with problems. But there are tens of thousands of years in the future. Our responsibility is to do what we can, learn what we can, improve the solutions, and pass them on.
... It is our responsibility to leave the people of the future a free hand. In the impetuous youth of humanity, we can make grave errors that can stunt our growth for a long time. This we will do if we say we have the answers now, so young and ignorant as we are. If we suppress all discussion, all criticism, proclaiming "This is the answer, my friends; man is saved!" we will doom humanity for a long time to the chains of authority, confined to the limits of our present imagination. It has been done so many times before.
... It is our responsibility as scientists, knowing the great progress which comes from a satisfactory philosophy of ignorance, the great progress which is the fruit of freedom of thought, to proclaim the value of this freedom; to teach how doubt is not to be feared but welcomed and discussed; and to demand this freedom as our duty to all coming generations.

" Cargo Cult Science http://calteches.library.caltech.edu/51/2/CargoCult.htm", adapted from a 1974 Caltech commencement address; also published in Surely You're Joking, Mr. Feynman!, p. 345
Context: All experiments in psychology are not of this [cargo cult] type, however. For example there have been many experiments running rats through all kinds of mazes, and so on — with little clear result. But in 1937 a man named Young did a very interesting one. He had a long corridor with doors all along one side where the rats came in, and doors along the other side where the food was. He wanted to see if he could train rats to go to the third door down from wherever he started them off. No. The rats went immediately to the door where the food had been the time before.The question was, how did the rats know, because the corridor was so beautifully built and so uniform, that this was the same door as before? Obviously there was something about the door that was different from the other doors. So he painted the doors very carefully, arranging the textures on the faces of the doors exactly the same. Still the rats could tell. Then he thought maybe they were smelling the food, so he used chemicals to change the smell after each run. Still the rats could tell. Then he realized the rats might be able to tell by seeing the lights and the arrangement in the laboratory like any commonsense person. So he covered the corridor, and still the rats could tell.He finally found that they could tell by the way the floor sounded when they ran over it. And he could only fix that by putting his corridor in sand. So he covered one after another of all possible clues and finally was able to fool the rats so that they had to learn to go to the third door. If he relaxed any of his conditions, the rats could tell.Now, from a scientific standpoint, that is an A-number-one experiment. That is the experiment that makes rat-running experiments sensible, because it uncovers the clues that the rat is really using — not what you think it's using. And that is the experiment that tells exactly what conditions you have to use in order to be careful and control everything in an experiment with rat-running.I looked into the subsequent history of this research. The next experiment, and the one after that, never referred to Mr. Young. They never used any of his criteria of putting the corridor on sand, or of being very careful. They just went right on running rats in the same old way, and paid no attention to the great discoveries of Mr. Young, and his papers are not referred to, because he didn't discover anything about rats. In fact, he discovered all the things you have to do to discover something about rats. But not paying attention to experiments like that is a characteristic of cargo cult science.

(From a 1963 letter to his wife Gweneth, written while attending a gravity conference in Communist-era Warsaw.)
"Letters, Photos, and Drawings," p. 90-91
What Do You Care What Other People Think? (1988)
Context: The real question of government versus private enterprise is argued on too philosophical and abstract a basis. Theoretically, planning may be good. But nobody has ever figured out the cause of government stupidity — and until they do (and find the cure), all ideal plans will fall into quicksand.

volume I; lecture 3, "The Relation of Physics to Other Sciences"; section 3-4, "Astronomy"; p. 3-6
The Feynman Lectures on Physics (1964)
Context: Poets say science takes away from the beauty of the stars — mere globs of gas atoms. Nothing is "mere". I too can see the stars on a desert night, and feel them. But do I see less or more? The vastness of the heavens stretches my imagination — stuck on this carousel my little eye can catch one-million-year-old light. A vast pattern — of which I am a part... What is the pattern, or the meaning, or the why? It does not do harm to the mystery to know a little about it. For far more marvelous is the truth than any artists of the past imagined! Why do the poets of the present not speak of it? What men are poets who can speak of Jupiter if he were a man, but if he is an immense spinning sphere of methane and ammonia must be silent?

"On the Philosophical Problems in Quantizing Macroscopic Objects"(ca. 1962-1963) as quoted by Morinigo, Wagner, & Hatfield, Feynman Lectures on Gravitation (2002)
Context: This is all very confusing, especially when we consider that even though we may consistently consider ourselves to be the outside observer when we look at the rest of the world, the rest of the world is at the same time observing us, and that often we agree on what we see in each other. Does this then mean that my observations become real only when I observe an observer observing something as it happens? This is a horrible viewpoint. Do you seriously entertain the idea that without the observer there is no reality? Which observer? Any observer? Is a fly an observer? Is a star an observer? Was there no reality in the universe before 109 B. C. when life began? Or are you the observer? Then there is no reality to the world after you are dead? I know a number of otherwise respectable physicists who have bought life insurance.

letter to Koichi Mano (3 February 1966); published in Perfectly Reasonable Deviations from the Beaten Track: The Letters of Richard P. Feynman (2005), p. 198, 201
also quoted by Freeman Dyson in "Wise Man" http://www.nybooks.com/articles/18350, The New York Review of Books (20 October 2005)
Context: The worthwhile problems are the ones you can really solve or help solve, the ones you can really contribute something to. … No problem is too small or too trivial if we can really do something about it. You say you are a nameless man. You are not to your wife and to your child. You will not long remain so to your immediate colleagues if you can answer their simple questions when they come into your office. You are not nameless to me. Do not remain nameless to yourself — it is too sad a way to be. Know your place in the world and evaluate yourself fairly, not in terms of the naïve ideals of your own youth, nor in terms of what you erroneously imagine your teacher's ideals are.

The Value of Science (1955)
Context: Of course if we make good things, it is not only to the credit of science; it is also to the credit of the moral choice which led us to good work. Scientific knowledge is an enabling power to do either good or bad — but it does not carry instructions on how to use it. Such power has evident value — even though the power may be negated by what one does with it.I learned a way of expressing this common human problem on a trip to Honolulu. In a Buddhist temple there, the man in charge explained a little bit about the Buddhist religion for tourists, and then ended his talk by telling them he had something to say to them that they would never forget — and I have never forgotten it. It was a proverb of the Buddhist religion:To every man is given the key to the gates of heaven; the same key opens the gates of hell.What then, is the value of the key to heaven? It is true that if we lack clear instructions that enable us to determine which is the gate to heaven and which the gate to hell, the key may be a dangerous object to use.But the key obviously has value: how can we enter heaven without it?

Source: No Ordinary Genius (1994), p. 251-252, from interview in "The Pleasure of Finding Things Out" (1981): video http://www.youtube.com/watch?v=NEwUwWh5Xs4&t=45m21s
(Also in book The Pleasure of Finding Things Out (1999) p. 23.)
Context: People say to me, "Are you looking for the ultimate laws of physics?" No, I'm not. I'm just looking to find out more about the world and if it turns out there is a simple ultimate law which explains everything, so be it; that would be very nice to discover. If it turns out it's like an onion with millions of layers and we're just sick and tired of looking at the layers, then that's the way it is!… And therefore when we go to investigate we shouldn’t pre-decide what it is we are trying to do except to find out more about it… My interest in science is to simply find out more about the world.

Part 5: "The World of One Physicist", "The 7 Percent Solution", p. 255
Surely You're Joking, Mr. Feynman! (1985)

Does not appear to be from any of his books or cited in a biography. A Google Books search shows that the oldest book citing "physics is like sex" is Scary Monsters and Bright Ideas (2000) by science broadcaster Robyn Williams. On p. 44, this book claims: "Einstein said, 'You do not really understand something unless you can explain it to your grandmother'. Richard Feynman added, 'Physics is like sex: sure, it may give some practical results, but that's not why we do it'." Given that Einstein didn't really say the former, it's likely that Feynman didn't really say the latter.
Disputed and/or attributed
Variant: Physics is like sex: sure, it may give some practical results, but that's not why we do it.

Source: QED: The Strange Theory of Light and Matter (1985), p. 24

On the numerical value of α, the fine-structure constant, p. 129
QED: The Strange Theory of Light and Matter (1985)

rather than with the event
Source: QED: The Strange Theory of Light and Matter (1985), p. 75-76

We haven’t any deep understanding of what we’re doing. If we tried to understand what we’re doing, we’d go nutty.
Source: No Ordinary Genius (1994), p. 236, from interview two weeks before his death in "The Quest for Tannu Tuva" (1989): video http://www.youtube.com/watch?v=Mn4_40hAAr0&t=51m49s