I sincerely hope I did not come off as insensitve.
This is just a theory I'm working on. To me, there are three types of thinkers, as stated in the original post. Linear thinkers are the ones who may not shine (in the office, let's say) but they are the ones happy to do the paperwork, take care of the real business, with no desire to rise above their position in life. That is not a fault, it's an assett.
Circular thinkers are linear thinkers with a mission. They combine their linear knowledge with a desire to learn more, and actively pursue that path to further education.
Spherical thinkers are those who see life in global terms. They have moved on from the circular, and can see things in 3D. There are no longer any simple answers, just vision.