Atle Selberg (Interview, June 11, 1989, page 30):
In some sense, I think those are probably the most important things, those that can be made simple.
Or, with one more phrase of context.
There are other things in mathematics that may seem impossible to being begin with, but after they have been done they seem very simple. In some sense, I think those are probably the most important things, those that can be made simple.
