(source) 
Richard P. Feynman
(11 May 1918  15 Feb 1988)

Richard P. Feynman Quotes on Theory (10 quotes)
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>> Click for 98 Science Quotes by Richard P. Feynman
>> Click for Richard P. Feynman Quotes on  Atom  Discovery  Imagination  Knowledge  Law  Mystery  Nature  Physics  Problem  Science  Thinking  Universe 
A good theoretical physicist today might find it useful to have a wide range of physical viewpoints and mathematical expressions of the same theory (for example, of quantum electrodynamics) available to him. This may be asking too much of one man. Then new students should as a class have this. If every individual student follows the same current fashion in expressing and thinking about electrodynamics or field theory, then the variety of hypotheses being generated to understand strong interactions, say, is limited. Perhaps rightly so, for possibly the chance is high that the truth lies in the fashionable direction. But, on the offchance that it is in another direction—a direction obvious from an unfashionable view of field theory—who will find it?
— Richard P. Feynman
As usual, nature’s imagination far surpasses our own, as we have seen from the other theories which are subtle and deep.
— Richard P. Feynman
Every theoretical physicist who is any good knows six or seven different theoretical representations for exactly the same physics. He knows that they are all equivalent, and that nobody is ever going to be able to decide which one is right at that level, but he keeps them in his head, hoping that they will give him different ideas for guessing.
— Richard P. Feynman
If there is something very slightly wrong in our definition of the theories, then the full mathematical rigor may convert these errors into ridiculous conclusions.
— Richard P. Feynman
It doesn’t matter how beautiful your theory is, it doesn’t matter how smart you are. If it doesn’t agree with experiment, it’s wrong.
— Richard P. Feynman
It is going to be necessary that everything that happens in a finite volume of space and time would have to be analyzable with a finite number of logical operations. The present theory of physics is not that way, apparently. It allows space to go down into infinitesimal distances, wavelengths to get infinitely great, terms to be summed in infinite order, and so forth; and therefore, if this proposition [that physics is computersimulatable] is right, physical law is wrong.
— Richard P. Feynman
Professor Bethe … is a man who has this characteristic: If there’s a good experimental number you’ve got to figure it out from theory. So, he forced the quantum electrodynamics of the day to give him an answer [for the experimentally measured Lambshift of hydrogen], … and thus, made
the most important discovery in the history of the theory of quantum electrodynamics. He worked this out on the train from Ithaca, New York to Schenectady.
— Richard P. Feynman
Shortly after electrons were discovered it was thought that atoms were like little solar systems, made up of a … nucleus and electrons, which went around in “orbits,” much like the planets … around the sun. If you think that’s the way atoms are, then you’re back in 1910.
— Richard P. Feynman
That was the beginning, and the idea seemed so obvious to me and so elegant that I fell deeply in love with it. And, like falling in love with a woman, it is only possible if you do not know much about her, so you cannot see her faults. The faults will become apparent later, but after the love is strong enough to hold you to her. So, I was held to this theory, in spite of all difficulties, by my youthful enthusiasm.
— Richard P. Feynman
The theoretical broadening which comes from having many humanities subjects on the campus is offset by the general dopiness of the people who study these things and by the Department of Home Economics.
— Richard P. Feynman
See also:
 11 May  short biography, births, deaths and events on date of Feynman's birth.
 Richard Feynman on The French Curve: a reflection on thinking inside the box.
 Genius: The Life and Science of Richard Feynman, by James Gleick.  book suggestion.
 Booklist for Richard Feynman.