I'm stealing this from another blog, but I just love it.
"He manipulated the work tremendously in the darkroom. He always said that the negative is the equivalent of the composer's score and the print is the equivalent of the conductor's performance, and the same piece of Mozart is conducted differently, performed differently, by different orchestras, different conductors, and Ansel performed his own negatives differently."
This blew my mind. He manipulated his negatives the exact same way we manipulate our RAW files in photoshop now.