"Recapitulating, I would like to stress that each of the two general attitudes, introversion and extraversion, manifests itself in a special way in an individual through the predominance of one of the four basic functions. Strictly speaking, there are no introverts and extraverts pure and simple, but only introverted and extraverted function-types, such as thinking-types, sensation types, etc."
Jung goes on to say that there could be sub-divisions in each function, e.g. intuitive and speculative, logical and mathematical, or empirical and positivist Thinking. He concludes:
"For the sake of completeness, I must add that I do not regard the classification of types according to introversion and extraversion and the four basic functions as the only possible one. Any other psychological criterion could serve just as well as a classifier, although, in my view, no other possesses so great a practical significance."
I for one can't wait to see what the next few years of neuroscience advances hold. Kahneman and Eagleman already talked about the brain's subconscious systems, and when I see him at the end of the month I'll hopefully remember to ask him if, then, our dominant and auxiliary functions would be located in the neurocortex while the inferior functions are buried deeper elsewhere.
And at the end of the day, is it our consciousness that defines our selves, or our unconscious predispositions? Are we more truly ourselves when we don't think about it?
Don't want to miss mentioning Dario's excellent work in this field either; in case you haven't already, check out his Google talk.