The thesis that humans start with a blank slate and their resource of knowledge is built up gradually through experience is how I usually approach SEO writing.

Say I get a client featuring a product, service or even location I know little about. I am, in effect starting with a blank slate, both on the page and in my mind. I will then pull from various resources to fill that page up with things I've gleaned from research on the internet as well as the knowledge I have gained over the years, combining them to write (hopefully) stellar content that not only pleases the client but gets them a lot of first page rankings as well!