Toni Morrison, The Art of Fiction No. 134, the Paris Review:

I had a very good editor, superlative for me — Bob Gottlieb. What made him good for me was a number of things — knowing what not to touch; asking all the questions you probably would have asked yourself had there been the time. Good editors are really the third eye. Cool. Dispassionate. They don’t love you or your work; for me that is what is valuable — not compliments. Sometimes it’s uncanny; the editor puts his or her finger on exactly the place the writer knows is weak but just couldn’t do any better at the time. Or perhaps the writer thought it might fly, but wasn’t sure. Good editors identify that place and sometimes make suggestions. Some suggestions are not useful because you can’t explain everything to an editor about what you are trying to do. I couldn’t possibly explain all of those things to an editor, because what I do has to work on so many levels. But within the relationship if there is some trust, some willingness to listen, remarkable things can happen. I read books all the time that I know would have profited from not a copy editor but somebody just talking through it. And it is important to get a great editor at a certain time, because if you don’t have one in the beginning, you almost can’t have one later. If you work well without an editor, and your books are well received for five or ten years, and then you write another one — which is successful but not very good — why should you then listen to an editor?