When asked, Jung said, that the opposite attitude of the function would be his guess from an interview. But that was off the cuff and he didn't place much thought in it since that wasn't his focus.
My guess is, that MBTI used it because those functions are opposites (the top two), they bring out the worst in each other (the dominant functions are at war), however, the worst would be classified as the unconscience of each type, or otherwise known as the shadow. Those last two functions indicates what the shadow is.
So in theory, the two types are supposed to "grow" together, like in therapy, by bringing out the worste of each other. But if both parties don't agree to it, or see the point or necessity of this process. it is doomed from the beginning.
Who said life is easy, those old couples still together today must have gone through some heavy stuff to make it through to the end. If only I can find an example.
Socionics, where the INTJ in MBTI maps 80% of the time to ILI-INTp makes the IEE-ENFp each other's Illusionary "partner"
Illusionary, or mirage, relations, like semi-duality, are similar to duality in that many of each partners' functions directly correspond to the unconscious expectations of the other. In relations of semi-duality partners' odd-numbered functions match those of the other person's dual, while in illusionary relations it is the even-numbered functions. That is, partners' use of their 2nd, 4th, 6th, and 8th functions more or less meets the other's expectations, while the rest are the opposite of what is expected for comfortable interaction. In practice this translates to a perception that the other person can be useful in solving practical issues that arise, but partners do not find each other fascinating â€as they do duals or semi-duals†due to an absence of suggestion through their 5th function.
At a distance, illusionary partners may experience a wide range of attitudes to each other â€from like to mutual mockery†but this is generally true of most intertype relations. In closer contact, partners find they can be of practical assistance to each other in a variety of ways, even if they are not driven to become close emotionally. Leadership duties are divided naturally between partners, one of which is extraverted and the other introverted, and who both share a common rational or irrational approach to living. Despite this significant underlying compatibility, prolonged interaction leads to a dissatisfaction with everything about the relationship that is related to the other's leading function. Partners unconsciously expect the other person to accept their general sentiments about things and build upon them, but illusionary partners inevitably present their own completely independent worldviews that are somewhat at odds with the other's. This is because if one partner's leading function is Si, the other's is Se; or, if one's is Fe, the other's is Fi, and so on. Partners' worldviews, central values, and general approaches are similar in that they focus on similar kinds of things, but they are hardly compatible in practice. Where one sees opportunities that must be developed immediately, the other wants to wait and do nothing for the time being. Then, the tables turn and the other person suddenly feels it is time to do something, while the other believes that there is nothing to be done at the moment.
Illusionary relations occur between two types, each of which has the others' "hidden agenda" (6th) function as their creative (2nd) function. However, one's leading (1st) function is the others' ignoring (7th) function. This means that the two are like a dual couple on how they interact with the world, but not on how they view the world. Illusionary relations tend to appear close or compatible from an outside perspective, but the partners themselves may not get along. The closer the relationship becomes, the more strained it gets. A parent-child illusionary relationship may be turbulent at home, but will be more natural, relaxed, and mutually beneficial when on vacation together.
Illusionary relations range from apparent compatibility, offset by the occasional small, short argument to mutual understanding and tolerance of the other's quirks to complete aversion and disregard for the other's lifestyle. The relationship usually progresses smoothly, but it depends on how close the partners are, what terms the relationship started on, and which of the two leads the relationship (usually the older, more experienced partner.) A lack of understanding of the other's lifestyle seems to be the biggest cause for upheaval. According to Filatova, the one in charge is important - if it is the more positive of the two, or the one more that is more of a natural psychologist, then the relation will run smoothly. Out of the four illusionary cases Filatova offers, two have the same types, but the one in charge is reversed. In the more successful relationships, the only dichotomy shared by the three types in charge is static. IEEs, which she says are life-loving optimists, are negativists.
There you have it, the IEEs are negativists.
MBTI also doesn't talk about function roles, instead it calls it "how it manifests", socionics assigns a role to it as the function is assigned to the type.
Short info on Socionics, Socionics uses all 8 functions and clearly defines the roles of each function. It also fixes the J/P for introverts, instead of using the first extraverted function to determine if it is J/P like in MBTI, it assigns it correctly (matter of opinion) by using the first function.
Then it also goes ahead and writes the functions correctly, so if INTJ (MBTI) is Ni Te Fi Se, it writes it Ni Te x x Se Fi x x. Now one can clearly see what the shadow is. Instead of MBTI where the it unconscience is written as Fi Se. The shadow of the INTJ (MBTI) or (ILI-INTp) is the ESFP. Socionics also writes the functions 1st 2nd 3rd 4th function as the conscience, but ordered from strong to weak, functions 5th 6th 7th 8th is the unconscience, this time however, ordered from weak to strong.
Duality in socionics means, each type's shadow is their dual partner.
Now what I find ironic about the whole MBTI thing is, that Isabel Myers (INFP) married a man named Chief, an ISTJ and a good man.
This means, in socionics it translates to EII-INFj (yes the J/P because of the dominant function actually being rational judging function) and the ISTp (again the dominant function is actually an irrational perceiving function). Turns out to be relations of Activity, one of the top 4 recommended relationships!
Activation, or activity, is an intertype relation between two people that belong to opposite poles of all basic dichotomies except for extraversion and introversion (e.g. SLI, which is introverted, sensing, logical, and irrational, and EII, which is introverted, intuitive, ethical, and rational.) Activation partners belong to the same quadra and thus find it comfortable to let down their guard around each other, making this relationship very easy to start.
Activation is a very common relationship for friendship. Activation is similar to duality in that each person provides those kinds of information that the other most expects, however, the emphasis is always somewhat different than subconsciously expected. Partners are able to provide each other with an abundance of useful information and assistance, but lead separate lives and make decisions based on criteria that don't seem too important to the other.
Activation partners who become close and discuss their strivings and personal worldviews often are struck by how radically different they are, despite the relative ease and benefit of communication. As opposed to duals, who tend to strive for the same things but from differing, though compatible angles, activation partners' approaches to achieving their goals tend to be fundamentally incompatible, due largely to the difference in rationality and irrationality. This means that while activation partners can talk and share their common hobbies with ease, they tend to view each other as separate entities with separate lifestyles and plans.
Then there is Lenore Thomson who's parents are still together is in a dual relationship. The irony never ends.