"...when I started writing the books, the first thing I had to decide was not what magic can do, but what it can’t do. I had to set limits on it immediately and decide what the perimeters are. One of the most important things I decided was that magic cannot bring dead people back to life. That’s one of the most profound things. The natural laws of death applies to wizards as it applies to Muggles and there is no returning once you’re properly dead..."
Okay here is the quote.

I just had to find it to show you where I got it from. Here is the link:
http://www.quick-quote-quill.org/art...iontransc.html
It is the 14th paragraph from the bottom.