We read from start to finish and front to back. However, most good writing isn’t created linearly. For instance, how can we write a really great introduction for an essay without having written down any of the details of the body? When we do this, chances are we have to go back and completely changed our introductions anyway. If you have an idea for an essay or a story you think really should come later in your piece, start with it anyway and you can always fill in the holes and return to your intro later. Starting from the middle or end and returning with fresh eyes to fill in the missing pieces will also allow you to see your work from different angles.