“One of the advantages of being over forty is that one begins to learn the difference between knowing and realising.”

Encountering Directors interview (1969)
Context: When a scene is being shot, it is very difficult to know what one wants it to say, and even if one does know, there is always a difference between what one has in mind and the result on film. I never think ahead of the shot I'm going to make the following day because if I did, I'd only produce a bad imitation of the original image in my mind. So what you see on the screen doesn't represent my exact meaning, but only my possibilities of expression, with all the limitations implied in that phrase. Perhaps the scene reveals my incapacity to do better; perhaps I felt subconsciously ironic toward it. But it is on film; the rest is up to you.

Paris Review interview (1958)

"The Making of a Scientist," p. 14 <!-- Feynman used variants of this bird story repeatedly: (1) "What is Science?", presented at the fifteenth annual meeting of the National Science Teachers Association, in New York City (1966) published in The Physics Teacher, volume 7, issue 6 (1969), p. 313-320. (2) Interview for the BBC TV Horizon program "The Pleasure of Finding Things Out" (1981), published in Christopher Sykes, No Ordinary Genius: The Illustrated Richard Feynman (1994), p. 27. -->
What Do You Care What Other People Think? (1988)
Context: You can know the name of that bird in all the languages of the world, but when you're finished, you'll know absolutely nothing whatever about the bird. You'll only know about humans in different places, and what they call the bird. … I learned very early the difference between knowing the name of something and knowing something.

The Salmon of Doubt (2002)
Context: For Children: You will need to know the difference between Friday and a fried egg. It's quite a simple difference, but an important one. Friday comes at the end of the week, whereas a fried egg comes out of a chicken. Like most things, of course, it isn't quite that simple. The fried egg isn't properly a fried egg until it's been put in a frying pan and fried. This is something you wouldn't do to a Friday, of course, though you might do it on a Friday. You can also fry eggs on a Thursday, if you like, or on a cooker. It's all rather complicated, but it makes a kind of sense if you think about it for a while.

Source: The Name of the Wind (2007), Chapter 6, “The Price of Remembering” (p. 49)

Uh-Oh: Some Observations from Both Sides of the Refrigerator Door (2001), p. 146
Context: One of life's best coping mechanisms is to know the difference between an inconvenience and a problem. If you break your neck, if you have nothing to eat, if your house is on fire, then you've got a problem. Everything else is an inconvenience. Life is inconvenient. Life is lumpy. A lump in the oatmeal, a lump in the throat and a lump in the breast are not the same kind of lump. One needs to learn the difference.

This is an advantage which scientists enjoy over most other people engaged in intellectual pursuits, and they enjoy it at all levels of capability. To be a first-rate scientist it is not necessary (and certainly not sufficient) to be extremely clever, anyhow in a pyrotechnic sense. One of the great social revolutions brought about by scientific research has been the democratization of learning. Anyone who combines strong common sense with an ordinary degree of imaginativeness can become a creative scientist, and a happy one besides, in so far as happiness depends upon being able to develop to the limit of one's abilities.
1960s, Lucky Jim, 1968