My favourite version of that: "You can edit crap. You can't edit blank."As (I think it was) Neil Gaiman said, you can't edit a story to make it better if you haven't written any of it down in the first place.
This is very true. In fact I've written a very successful five-part series that has spent years on the NYT bestseller list and is currently being optioned for a HBO adaptation, all inside my mind.Besides, you can edit something you haven't yet written down. You don't need a piece of paper (or a computer screen) with writing on it in order to edit and revise. Much of writing can take place entirely inside the writer's mind, so that when they do sit down too the physical task of writing, most of the editing and revising is already done.