Writing goes all ways: forwards, backwards, sideways, over there, and over here. In fact, the only piece of the writing process that occurs at a set point in time is publishing.
Writing is a process. It is recursive. No piece of writing is ever "final." Something can always be better. I often feel this way whenever I read back over my own old essays and inevitably find a sentence that could be better, a paragraph that could be stronger, or a word that could be more precise.