Additionaly, due to extravert bias, they got the P and the J the wrong way around when designing the 4-letter codes:
- "I" means your top function is introverted (this makes sense)
- But "P" means your top extraverted function is a "P" function (N or S) rather than a "J" function (F or T)
But as an introvert's top function is introverted not extraverted, and according to the theory J/P alternates down the stack, that mean's that an I..P type's overall top function is a actually "J" function. Specifically in the this case "Introverted Thinking" (often notated "Ti").
Introverted Thinking is actually slow in two ways:
- Because it's a J ("Judgement") function. Which just means it's either an F ("Feeling" or T ("Thinking") function. These correspond to the Slow thinking from Kahneman's Thinking Fast and Slow. That's what "Judgement" is.
- Because it's an I ("Introverted") function. This doesn't make it actually slow, it just makes it appear slow. This is because "I" functions involve a relatively long amount of internal coginition before seeking more input from the external environment (this is what it means to be an "I" function). Which will appear slower to the observer who can't observe the persons internal thinking processes and can only observe when they interact with their environment.
Taking your description naively, such a person would likely be an INTJ. Which has a functional stack of (in order of most to least used thinking processes): Introverted Intuition - Extraverted Thinking - Introverted Feeling - Extraverted Sensation.
I realise that doesn't make much sense if you are used to applying the 4-letter code to persons directly. But as I mentioned above, I don't think that is a good way of doing things. It makes more sense if you take the 4 letter codes as simply being an opaque shorthand encoding for functional stacks (with the individual letters in the code not having meaning on their own, only indirectly through their correlation to a functional stack). E, I, N, S, T, F, P, J then become properties of thinking processes and an "extraverted person" is simply someone who primarily tends towards using one of the four extraverted thinking processes (Fe, Te, Ne, Se). They may or may not present like the everyday understanding of how an extravert presents (e.g. sociable and chatty). Typically people who have a "Extraverted Feeling" (Fe) dominant function closely fit this stereotype, but other types extraverts often don't.
If someone did the test and got INTP, then I think it is fair for them to describe themselves as INTP. You might say that the test was wrong, but as I said that isn't how most see MBTI.
It's also what the more advanced "Step III" version of the official MBTI test is based on. But that that assessment isn't just a simple questionaire: it can be only be conducted by a trained assessor and the cost of training course runs to thousands of pounds, so most people don't have experience of it, and it ends up being the simpler naive model that most people learn about.
Why? N stands for intuitive, and the rest have nothing to do with thinking fast or slow. P just means you don't judge quickly, P could still come up with tons of ideas and explanations quickly, they just don't judge one of those correct.