Book excerpt: Spotting a face in a crowd is so easy, you take it for granted. But how you do it is one of science's great mysteries. Vision is involved in nearly a third of everything a brain does and explaining how it works reveals more than just how we see. It also tells us how the brain processes information – how it perceives, learns and remembers. In We Know It When We See It, pioneering neuroscientist Richard Masland covers everything from what happens when light hits your retina, to the increasingly sophisticated nerve nets that turn that light into knowledge, to what a computer algorithm must be able to do before it can truly be called ‘intelligent’. It is a profound yet accessible investigation into how our bodies make sense of the world.