You really want to go in the solipsist direction? There's nothing to be found there.
(Heh, I picked on this one, and then saw Jim did too.)
It's not solipsist, that's the thing. Just introverted. Subjective. The question of how cognition in an introverted attitude is not moribund is a good one. Jung's answer was that balancing functions appear. But it remains true (aka, theory sez) that cognition in an introverted attitude is mental working that, at a minimum, rejects immediate structuring that would otherwise be drawn from the external world. The cognition spirals off on its own path and generates its own structuring. And every one of us is familiar with this because every one of us does it to some extent. The answer to why all that is normal, capable, and productive, and not (automatically) solipsist, is found in some proper discussion of what it is to have and/or be a product of a dynamic system of mutually interdependent cognitive functions. Somewhere in that discussion would be some description of one or the other of how the same activities are performed (and transformed) by different functions in different people and how all people use the same functions to do the same thing regardless of their supposed function order and strength.
Not to derail too much, but doesn't Te inform you though? It can't be merely a volitional function.. or is it? It must be opening up your perspective to other ideas somewhat? And if it doesn't, then how could you use it?
While a person on an Ni jag is spiralling off into his own warlock world, Se is still there gawping at the surroundings and believing everything. There's something about Se being present as an unconscious foundation for the conscious functioning of Ni that means introversion isn't just introversion, and no Ni is pure Ni. Jung said as much when he spoke of pure functions. He said something along the lines of, the purest, best expression of the rejection of objectivity is the null, and a most fundamental rejection of subjectivity would be the complete, exact and essentially personless mirroring of the external world.
Something about unconscious functioning.