I have to make this comment, considering something Super said about people not knowing them as well as they think they do.
Ok, perhaps in 98% of an NTs daily interactions, this would hold true. I know there's all sorts of things running through my INTJ best friend's head, but honestly, I've met NFs with much crazier private thought lives who have shocked the heck out of me with some of their revelations. I'd say MOST people with a profound sense of the world will be "unknowable" to many degrees.
Having said this, an ENTP I knew, have known well for years, used to get so ticked with me when I could predict him or understand instinctively what he needed. He'd go storming away with "You don't know me! Nobody does!" and I'd just go back to whatever I was doing before without comment. I knew he'd return at some point, and when he did, it would be quietly and sheepishly, still hanging on to a little of the vexation, but I think it had more to do with the frightening idea that someone *could* get him without effort. It was perhaps his most private nightmare that I could simply walk into his sacred internal places without being harmed or delayed by his security systems. Not that I would intrude on him. I'm simply immune to his barbs, like a clown fish in an anemone. He was practically in a sweating panic about it.
The enshrined hypothesis of being The Unknowable, I've encountered this with many NTs, and have simply sidestepped it or let it ride, figuring whatever I've gleaned from their behavior, body language and other data will either steer me in the right direction or they'll actually break down and TELL me something they want me to know. Otherwise, I just wait and accumulate. They hate to believe they're telegraphing ANYTHING to the outside world, an illusion of supreme control. Sure, they make fantastic poker players. But they're not air-tight, no matter how much they'd like to believe it.