The key thing to realise, Pinker argues, is that writing is "cognitively unnatural." For almost all human existence, nobody wrote anything; even after that, for millennia, only a tiny elite did so. And it remains an odd way to communicate. You can't see your readers' facial expressions. They can't ask for clarification. Often, you don't know who they are, or how much they know. How to make up for all this?
Pinker's answer builds on the work of two language scholars, Mark Turner and Francis-Noël Thomas, who label their approach "joint attention". Writing is a modern twist on an ancient, species-wide behaviour: drawing someone else's attention to something visible.