"Nonracers. The emptiness of those lives shocks me." - THE RIDER
If you're someone who is happy being completely absorbed by one field, I've labeled you a 'Diver.' Some clear examples of Divers are professional musicians, scientists, mathematicians, professional chess players, athletes, business owners and financiers. These people may 'relax' with a hobby, but they're rarely passionate about anything but their field. In fact, Divers often wonder how people can be interested in anything but what they're interested in. Sometimes they even make fun of themselves for it, like the racing bicyclist Tim Krabbe described in The Rider, who glances up from his gear to look at people walking and says, "Nonracers. The emptiness of those lives shocks me."
After a little googling I discovered Tim Krabbe himself is an ACTUAL Renaissance Man. He's a journalist, novelist, champion chess player AND cyclist.
The character in his book is who Sher is referencing as a 'Diver.'
By contrast, Scanners rarely think what other people are doing is empty. They're always curious to know 'what's out there' and love to poke their noses into just about anything. A Diver rarely spends a moment wondering what he might be missing when he's totally absorbed in his field. On the other hand, 99% of Scanners spend a lot of time scanning the horizon, thinking about their next move.