Okay, first comes the person, and then comes the type. There is no chicken and egg thing here. We were not all created with a "type" first, with the preceeding events obscuring this "original type", so that if we look deep enough we can find what we were originally "made" as. No, it goes biology first and formost. Your chemicals make you who you are, and the type is a general conception of what happens with people in general.
(In English: Types aren't "real". The only "real" thing is the abstract system or language by which we talk about type, and the differences come in how we figure this out and apply it in reality.)
People of a certain type are not exactly the same. Different processes may have created the same "type", who knows? One person may be an N type because of general lack of testosterone or something like that, where another person may be an N type because of over/under-connectiveness of synapses, or even both. One person may be N because of lack of S, or S because of lack of N.
One person may be a T-type because of total lack of the emotional center in the brain, where another person can be a T-type simply because they love to think and can't get enough of it.
These reasons are vast and varied, and Socionics ties all this together by creating mutually exclusive dichotomies by which we can categorize all these different ways that people arrive at the same general "typology". This is why there must be a seperation between things like what a "function" is in the abstract, and what people who "have" a certain function tend to "do", because who's to say that these different "paths" to the same type aren't different in important ways?
People wonder why VI is a part of type identification. Duh. Your psyche is determined by biology the same way your body is, and to say otherwise would just be silly! Obviously, you can have different interpretations based off of VI, but fundamentally, the information about what chemicals circulate in your body, hormones and junk, should be there to a certain extent. This is what VI is based on. It isn't magic.
There are some basic and important categorizations that have alot to do with "type", but it isn't like this is the only way we can categorize people. I could easily make a "gayness" dichotomy and add that on to it, and have Extroverted and Introverted "Gayness" (complete with VI, and we all know this is possible), but this is not what Socionics is, so it would be better left to another subject.
When these dichotomies are combined, "type" is created:
Objective/Subjective (E/I) - Objective is something that is more or less agreed upon, where subjective is more up to the person themselves.
Actual/Potential (S/N) - Actual things that exist, vs. things that don't exactly need to "exist".
Logical/Emotive (T/F) - Duh.
Rational/Irrational (J/P) - Determining something if and only if it is determinable vs. determining something whenever it is needed. (LOL, gotta work on this one.)
It appears that a person has to prefer one or the other, if only just a little bit, and what results is type. The "functions" come from figuring out what happens when you combine these Dichotomies, say Objective Sensing, or Subjective Ethics.
SO, you can say that an INFj type is used to thinking Rationally, Subjectively, Intuitively, and Emotionally. Thus, they cannot have an Irrational "function" be the main method of thinking, because that is not a "rational" function that "determines something", it is an "irrational" function.
Of course, calling J/P just being responsable is silly, it has to do with a basic level of how you think: organization of thought comes with a price, you have to be able to cut off lots of things, alternatively, taking everything into account comes with a price, and that is your thoughts become disorganized.
The rest, the things that follow, vary greatly. The problem becomes to what extent is Socionics exactly correct or not. What are the limits to what we can say with this idea of "type"? The limits of this system are actually greater then we think.