[TL;DR] MBTI mixes up all the introvert types. Instead of letting all four Jungian dichotomies I-ntroversion / E-xtraversion, S-ensing / i-N-tuition, F-eeling / T-hinking, P-ercieving / J-udging determine the two main functions (primary and secondary); it just assumes that the dichotomies determine what the extraverted main function should be, and whether the primary function is introverted or extraverted. Regardless of whether the person is an introvert or extravert in regards to point 1. It thus effectively directly disregards whether the person is judging or perceiving, and only indirectly takes it (wrongly) into account by happenstance via Introversion/Extraversion as primary function.

EDIT: This criticism is implicitly derived from Socionics. I have discussed Socionics explicitly in relation to this exact criticism and subject matter on PPD before, but I should have been more explicit and clear about this time too. I just didn’t want it to be about forcing another theory on MBTI adherents, but rather about how MBTI can be improved on its own terms. Thus the lack of focus on Socionics.

ISFP example:

1) The type is Perceiving (Sensing or iNtuition dominant, these being the perceiving functions). The type’s preferred perceiving function is Sensing, so one of the main functions has to be extraverted sensing.

2) Type is introverted thus the primary function is introverted, excluding extraverted sensing meaning it must be the secondary function. This leaves the primary function determined to be Introverted Feeling, along with Extraverted Sensing as secondary function.

Fi - Se

How ISFP should really be determined:

1) The Jungian dichotomies describe the primary, dominant function, as was Jung’s original focus, and only secondarily what the secondary function is (duh): Whether the primary function is Introverted or Extraverted, iNtuitive or Sensing, Thinking or Feeling, Perceiving or Judging.

2) The type is Introverted and Perceiving. The dominant Perceiving function is Sensing. Thus the type’s primary function is Introverted Sensing (relaxation, ambience, bodily sensation, good taste, aesthetics, clothing, fabric). The remaining preferred function is Feeling, which is Extraverted, balancing out the primary introverted function as secondary function.

Introverted Sensing - Extraverted Feeling

Si - Fe

Who is the real ISFP Si-Fe?

Relaxed, dresses well, comfortably (Si), socialite, knows how to arrange social parties (Fe) with a relaxing atmosphere (Si), with a wide array of sensory pleasures (Si) – food, drink, lighting, music – but still laidback and socially somewhat reserved. Very good at listening to the body and taking bodily signals seriously (Si) Possibly interior decorator or studying aesthetics. Appreciates social mood and social, emotional expressions of enjoyment (Fe) over deep, implicit personal connections (Fi). The latter drives the former to the foreground, but remains in the background.

Now, who is the real Fi-Se? (real ISFJ)

Strong estimation of moral character in other people (Fi). Knows how to set the right psychological distance (Fi). Strong (Se) but fair and correct estimation of other people’s character (Fi). Sees right through insincerity and falseness (Fi).

Slightly traditional moral views (Fi and preferred Sensing combined). Well-developed will-power (Se) and quite social, but ultimately relaxed and thriving in own company. Can use the right and appropriate amount of aggression and persuasion (Se) towards other people. Appreciates deep personal connections (Fi) over enjoyment over social emotional expressions (Fe). The latter drives the former to the foreground, but remains in the background.