Reckful On Type Dynamics
It's not hard to find MBTI forum posters who'll tell you that Jungian/MBTI type is basically all about the functions, and that the dichotomies mostly just deal with superficial stuff, and that you should think of them primarily as "letter codes" that need to be decoded to lead you to the deeper stuff.
But reckful's here to tell you that those people have been taken for a ride. Not even Jung himself prioritized the so-called "cognitive functions" in the way that a lot of those confused forumites do. In fact, Jung spent more of Psychological Types talking about the things he thought extraverts had in common and introverts had in common than he spent talking about all eight of the functions put together; and in the Foreword to a 1934 edition of the book, he bemoaned the fact that too many people were inclined to view Chapter 10 (his function descriptions) as the essence of the book, while noting that he'd stuck those at the back of the book for a reason.
And in any event, and regardless of what Jung's perspective was, it's been close to 100 years since Psychological Types was published, and a lot of studies have been done since then — and it shouldn't surprise you to hear that the modern MBTI reflects countless adjustments and improvements to Jung's original concepts. Mystical streak notwithstanding, Jung was a believer in the scientific approach, and Isabel Myers took Psychological Types and devoted a substantial chunk of her life to putting its typological concepts to the test in a way that Jung never had, and in accordance with the psychometric standards applicable to the science of personality. Myers adjusted Jung's categories and concepts so that they better fit the data she gathered from thousands of subjects, and by the start of the 1960s (as the leading Big Five psychologists have acknowledged), she had a typology that was respectably tapping into four of the Big Five personality dimensions — long before there really was a Big Five. And twin studies have since shown that identical twins raised in separate households are substantially more likely to match on those dimensions than genetically unrelated pairs, which is further (strong) confirmation that the MBTI dichotomies correspond to real, relatively hard-wired underlying dimensions of personality. They're a long way from being simply theoretical — or pseudoscientific — categories with no respectable evidence behind them.
You may have heard that Myers "dumbed down" Jung for "the masses," or turned Jung's typology into crude stereotypes for testing or job-placement purposes, or bla bla bla, but those are just uninformed memes that get passed around the Great Internet Forum Echo Chamber. If it wasn't for Myers, chances are pretty good that few of us would ever have even heard of Psychological Types, which was largely ignored until Myers did the necessary data-gathering to figure out how the personality characteristics that Jung had focused on actually clustered in real people.
Jung included what's arguably the lion's share of the modern conception of S/N (the concrete/abstract duality) in his very broad notion of what E/I involved. But Myers discovered that there are abstract extraverts (ENs) and concrete introverts (ISs), and that there's no significant correlation between Myers' (statistically supportable) versions of E/I and S/N. Jung said extraverts tend to subscribe to the mainstream cultural views of their time, while introverts tend to reject mainstream values in favor of their own individualistic choices. But Myers discovered that a typical ISTJ is significantly more likely to be a traditionalist than a typical (more independent-minded) ENTP. Jung said an extravert likes change and "discovers himself in the fluctuating and changeable," while an introvert resists change and identifies with the "changeless and eternal." But Myers discovered that it was the S/N and J/P dimensions that primarily influenced someone's attitude toward change, rather than whether they were introverted or extraverted.
And so on. The appropriate way to view the Myers-Briggs typology is not as some kind of "dumbed down" — or simplified (or more "testable") — version of Jung's original typology. Instead, it's fairer to say that the Myers-Briggs typology is basically where Jung's typology ended up after it was very substantially modified — not to mention expanded — to fit the evidence. And if you're interested, you can read quite a lot about the scientific respectability of the MBTI — and about several other issues often raised by people claiming to "debunk" the MBTI — in a couple of the posts linked at the end of this one.
Buuut contrary to what some of the function aficionados would have you believe, the scientifically respectable side of the MBTI is the dichotomy-centric side — and the dichotomies differ greatly from the so-called "cognitive functions" in that regard. The functions — which James Reynierse (in "The Case Against Type Dynamics") rightly characterizes as a "category mistake" — have barely even been studied, and the reason they've barely been studied is that, unlike the dichotomies, they've never been taken seriously by any significant number of academic psychologists. Going all the way back to 1985, the MBTI Manual described or referred to somewhere in the neighborhood of 1,500 MBTI studies, and as I understand it, not one of the many study-based correlations reported in the manual were framed in terms of the functions. The third edition of the MBTI Manual was published in 1998 and, as Reynierse notes in that same article, it cited a grand total of eight studies involving "type dynamics" (i.e., the functions model) — which Reynierse summarizes as "six studies that failed, one with a questionable interpretation, and one where contradictory evidence was offered as support." He then notes: "Type theory's claim that type dynamics is superior to the static model and the straightforward contribution of the individual preferences rests on this ephemeral empirical foundation."
And contrary to the notion that a function-centric perspective offers more richness and depth than a (properly framed) dichotomy-centric perspective, and as further discussed in one of the posts linked at the end of this one, it's actually the dichotomy-centric perspective that's richer and more flexible.
If you're interested in reading more about that Harold Grant function stack, and about the relationship between the dichotomies and the functions, the place of the functions (or lack thereof) in the MBTI's history, and the tremendous gap between the dichotomies and the functions in terms of scientific respectability, you might want to consider having a cup of coffee and working your way through some or all of the following posts: