In mathematics you don't understand things. You just get used to them.

I read this quote the other day, and thought it was awesome. It's by John von Neumann, and according to wikiquote (and therefore true) this was a,

Reply to Felix T. Smith who had said "I'm afraid I don't understand the method of characteristics." —as quoted in The Dancing Wu Li Masters: An Overview of the New Physics (1984) by Gary Zukav footnote in page 208

There are some other great quotes there, like

You should call it entropy, for two reasons. In the first place your uncertainty function has been used in statistical mechanics under that name, so it already has a name. In the second place, and more important, no one really knows what entropy really is, so in a debate you will always have the advantage.

Truth is much too complicated to allow anything but approximations.

and a relatively well known one:

You wake me up early in the morning to tell me that I'm right? Please wait until I'm wrong.

I also want to point out the quote about him, by George Pólya:

There was a seminar for advanced students in Zürich 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 didn't 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.

The only place I had hear of him before was because the Neumann function (actually a particular Bessel function) turned up in the solution to the Schrödinger equation for the hydrogen atom.

And just for fun, lets see if anyone can get this sequence:

0.3, 0.4, -0.1, -0.5, -0.8, -2, -4, -10, -60, -300, -2000

Hint: these are all rounded to 1 significant figure as the decimal bit is nasty (probably irrational)