INFJs are often said to be people pleasing extraverted Feelers, but as one observant ISTP had once pointed out on a forum, INFJs are also rebels. It stands to reason why we are associated with counterculture. The ISTP poster saw a similarity in the two types, INFJ and ISTP; curiously, the post was under a thread about ISTPs being mistyped as INFJs. Based on quick observation, the two are almost complete opposites, the INFJ emotionally sensitive and the ISTP as detached as an INTP, but the types share the Ni-Ti tertiary loop, meaning they have the same exact preferences but in different order (the other two that share the same top four preferences are ENFJ and ESTP who share the Fe-Se loop). So how similar are we, really?
INFJ and ISTP are both philosophical types. INTJs are more known for their inclination towards philosophical pursuits, while INFJs are more often associated with psychology (because as NF (iNtuitive Feelers), they're credited with being experts at character study, like the ENFJ Actor), but the fact that clergy is among the INFJ's suggested careers is with good reason; we ask the deep questions. It turns out INFJ and ISTP share a tendency to cope through philosophical means, "with ISTPs ranked first among all the types in using spiritual /philosophical coping resources." (Source: J. Shelton, in MBTI Applications by A.L. Hammer, 1996. I don't ever recall the ISTP being called a philosophical type before, but there it is--the tools they try to master are not only in the physical realm. Beliefs and belief systems can be useful tools in many cases; what we choose to believe has transformative powers.
Now, I don't know if using exaggerated forms of MBTI types as psychological disorders is the most helpful way to figure out one's type, but since that's the concept behind the Enneagram, I suppose this method can help in some cases to gain additional insight. According to this article I found: (Know Thyself--theINFJcafe, (quoted from site), "there's a reason that types don't have BOTH top functions introverted (or extraverted). Why? The result is an unbalanced psyche. The primary job of the secondary function is to balance out the dominant.
Here's a link to the article: Dominant/ Tertiary Loops and Common Personality Disorders (my note: link on page is broken)
"The article suggests that whenever the secondary function is suppressed, underdeveloped or not given value, the individual can display characteristics of documented personality disorders. Specifically for the INFJ, a well-developed Fe is required to balance out our introverted intuition and thinking. Suppressing the Fe or ignoring its value can, according to the article, put us in a state resembling Schizoid Personality Disorder. Same for the ISTP, whose Ti/Ni loop is the reverse of ours:
"I found this fascinating. I remember coming across a description of Schizoid Personality Disorder once and laughing uneasily--because it came close to describing my personality when I'm under a great deal of stress. I tend to retreat into myself when things aren't going well, and my Fe/Se aren't able to work properly to balance out the internal thinking and introspection."
So, not only did that article get me thinking about the philosophical nature of INFJ and ISTP types, it also reinforced something I already knew about the importance of getting enough exercise, be it physical, mental, or emotional. I reflected on how young boys often get in trouble for the simple lack of getting enough physical exertion, frustration getting built up, leading to thoughtless actions from being bored and restless. As a creative type, I can't imagine a life where I couldn't express myself in some way, because that is my form of communication. Could an artist go mad from not being able to create, like the restless youths need extracurricular activities to burn off steam? I don't know, but it sounds logical that we rational animals have a variety of needs, as well as specific needs based on individual preferences to feel healthy and whole. Sometimes, people need pure enjoyment, too, as frivolous a need as it seems.
I could see how based on the above information that someone might think that the people who can't get themselves to fit one type easily must be "unbalanced," but I don't think that's necessarily the case. The MBTI is not useful for diagnosing disorders, and no one should be rushed or pressured into committing to one type for fear of seeming unbalanced. What's interesting to me is that by building on Jung's original cognitive function theory which was simpler, grouping types based on the dominant cognitive function (INTJ /INFJ are essentially the same type based on Ni, likewise ISTP / INTP based on Ti), the Myers-Briggs system has given us new insight into how our secondary preference can deeply affect our needs which, when unfulfilled, can affect our overall psyche.
So, now I can ask myself which my greater need is, that of fulfilling my Fe or Te (extraverted Thinking), to determine between INFJ and INTJ types. It's unclear to me what it means to need to fulfill my Fe more than other types do (except the ISFJ for whom Fe is a secondary function), but I can vouch for my need to feel something strong in order to feel inspired to create something and to feel fully alive.