*scratches head*
That's what it is. Briggs read about Jungian views, talked to him and wanted to create a test for it. Jung said "that's not how I work", and so Briggs created a inventory for it. They tested, worked on it, etc... Eventually it drew upon the scientific method and they created step II - factor analysis, sub traits, reliability studies.
The instrument itself was designed to find a way to type individuals. The progress of the instrument has been from Jungs approach to a scientific approach.
(Maybe the word "validated" is throwing up the wrong message here. When I say valid, I'm using the clinical definition - that it actually reflects something meaningful - functions if you wish, but behaviour or underlying facets of personality if not. Of course, there are those that argue about the validity of a lot of instruments, and I would be among them, but as far as it goes, MBTI isn't a lightweight in this area.)