"A mathematical theory is not to be considered complete until you have made it so clear that you can explain it to the first man whom you meet on the street." -Hilbert

I enjoy explaining maths I've learned to strangers, and admire those who are able to do so well.

I also enjoy this, though it is perhaps a little harsh:

"If one were to bring ten of the wisest men in the world together and ask them what was the most stupid thing in existence, they would not be able to discover anything so stupid as astrology."-Hilbert