tunnel (RT) is one of many alternative, self-sufficient
ways of looking at the world. It is superordinate
to a belief system
(a RT can contain more than one BS), and subordinate
to a mind
(a mind can have more than one RT)
Most people have only one reality tunnel. They have one self-sufficient system of viewing the world. But it is possible, and very useful (if you can pull it off) to build in your mind several alternative reality tunnels.
An RT is different from a belief system in that RTs are by necessity completely inconsistent with each other. For instance, I keep two basic tunnels in my head -- one that is skeptical/scientific/rational and another that is mystical. The things that it is necessary for me to build into my mind as a scientist, (necessity of proof, abstention from wishful thinking to avoid clouding experimental data, etc.) are inconsistent with the things I need to in order to function spiritually (wonder at the connectedness of life, love for living things, etc.) Keeping them distinct makes both of them work better. I switch back and forth depending on what the situation/problem calls for.
Of course, to avoid losing your mind to multiple personality disorder, you need some way to integrate these into one mind. The answer is a meta reality tunnel -- a tunnel that acknowledges the existence of other tunnels and knows when each will be useful.
So, after all that, we have a basic tree: The mind branches into meta-tunnels (usually only one is necessary), each of which branches into tunnels, each of which branches into belief systems.
You computer science types may understand better if I say that the mind is the whole computer, the meta-tunnel is LILO, the tunnel is the operating system and the belief system is the window manager. All of them draw from the same set of hardware, (but some may be partitioned) and coexist in the same computer.