(source) 
John von Neumann
(28 Dec 1903  8 Feb 1957)

Science Quotes by John von Neumann (4 quotes)
Neumann, to a physicist seeking help with a difficult problem: Simple. This can be solved by using the method of characteristics.
Physicist: I'm afraid I dont understand the method of characteristics.
Neumann: In mathematics you don't understand things. You just get used to them.
Physicist: I'm afraid I dont understand the method of characteristics.
Neumann: In mathematics you don't understand things. You just get used to them.
— John von Neumann
Anyone who considers arithmetical methods of producing random digits is, of course, in the state of sin.
— John von Neumann
The sciences do not try to explain, they hardly even try to interpret, they mainly make models. By a model is meant a mathematical construct which, with the addition of certain verbal interpretations, describes observed phenomena. The justification of such a mathematical construct is solely and precisely that it is expected to work—that is, correctly to describe phenomena from a reasonably wide area.
— John von Neumann
What is important is the gradual development of a theory, based on a careful analysis of the ... facts. ... Its first applications are necessarily to elementary problems where the result has never been in doubt and no theory is actually required. At this early stage the application serves to corroborate the theory. The next stage develops when the theory is applied to somewhat more complicated situations in which it may already lead to a certain extent beyond the obvious and familiar. Here theory and application corroborate each other mutually. Beyond lies the field of real success: genuine prediction by theory. It is well known that all mathematized sciences have gone through these successive stages of evolution.
— John von Neumann
Quotes by others about John von Neumann (3)
The spectacular thing about Johnny [von Neumann] was not his power as a mathematician, which was great, or his insight and his clarity, but his rapidity; he was very, very fast. And like the modern computer, which no longer bothers to retrieve the logarithm of 11 from its memory (but, instead, computes the logarithm of 11 each time it is needed), Johnny didn't bother to remember things. He computed them. You asked him a question, and if he didn't know the answer, he thought for three seconds and would produce and answer.
There was a seminar for advanced students in Zόrich that I was teaching and von Neumann was in the class. I came to a certain theorem, and I said it is not proved and it may be difficult. Von Neumann didnt say anything but after five minutes he raised his hand. When I called on him he went to the blackboard and proceeded to write down the proof. After that I was afraid of von Neumann.
Running overtime is the one unforgivable error a lecturer can make. After fifty minutes (one microcentury as von Neumann used to say) everybody's attention will turn elsewhere.
See also:
 28 Dec  short biography, births, deaths and events on date of Neumann's birth.
 John von Neumann  context of quote In mathematics you don't understand things.  Medium image (500 x 350 px)
 John von Neumann  context of quote In mathematics you don't understand things.  Large image (800 x 600 px)
 John von Neumann: The Scientific Genius Who Pioneered..., by Norman MacRae.  book suggestion.