It has been said by some that iNtuition cannot be described in words, but I say nay! Any psychological phenomenon can be described. After years of reflection, I have put together a conceptual understanding of what introverted intuition is and entails.
When you talk to an INFp, they are aware that you are talking to their persona. They consciously (on some level) sculpt that persona, according to their social environment. This makes an INFp very aware of their own projection and cognitive dissonance in the social world. They never adhere to any idea (social norms included) without question; this is because deep down, they compare information almost inadvertently with time, which, if you think about it, is really the only constant thing in the world. It is the most constant (if not reliable) point of reference; this could have something to do with why Ni's are consistently some of the most intelligent and adaptable of the types (INTps are statistically the most intelligent type). Some Ni's are more apt to express their deeper convictions, but confidence really varies among individuals. Ni's certainly have a greater capacity for confidence. Ni is a powerful function, but it also needs lots of information to grow. The more of the world an Ni sees, the more they realize that they can hold in their heads; an Ni can use their imagination to play through scenarios with ease, and from a very holistic viewpoint. Essentially, the more they experience, the more they realize they are capable of imagining. INFps often use their imaginations to premeditate social encounters; the movies in their heads are often uncannily accurate because, by understanding the mechanisms by which people adhere to social conventions, they can capture the essence of a person and most of their nuances, almost entirely objectively. They actively seek out their own predispositions and attempt to be rid of them, so their imaginations will be more accurate. Deep down, an Ni has a judgment of you, but depending on how much stake they put in social relationships, they may or may not voice their criticisms. If an Ni has been raised in such a family that they learn they have no way to turn, nobody to fall back on, they may never realize the nature of their true opinions toward people; they are slaves to their persona, and they know it. Cognitiveprocesses.com says about extraverted feeling, "Often with this process, we feel pulled to be responsible and take care of others’ feelings, sometimes to the point of not separating our feelings from theirs." They may wonder which part of them is their real self, when it is in fact the part that wonders. A mentally strong INFp can be as passionate or dispassionate as they choose, since they understand the expected interpretations of, and reactions to, comic and dramatic situations. Often they can bullshit their way halfway through a conversation by laughing where they ought to (Ni applied to Fe gives a sense of how a person will react to something, and so when they will laugh), and then catch up. This isn't to say that they aren't affected at all, deep down, by others; they are one of the most sensitive types (INFj is arguably the most sensitive). They may, however, be moved for different reasons. INFps gather expected interpretations by listening closely to people's conversations, but especially the descriptors they use. Often fantastic at poetry, they pick up on differences in the ways individuals interpret things and hold them somewhat consciously in their minds; this makes them excellent in mixed company, especially if they've thought about the situation beforehand.
I suspect that the prefrontal cortex embodies Ni. Among other things (executive functions, control of emotions), the prefrontal cortex is a virtual reality simulator. I also suspect that there is a direct biological relationship accounting for the mutual exclusion of Ni and Si in the ego. My biggest idea is that perhaps, since through our evolutionary timeline, Si (physical memory) was one of the first things to develop (necessary to act after the fact, and to store that action in case it happens later, so one can react faster), and Ni has come only lately (which would make sense if the prefrontal cortex embodies Ni, because the brain has evolved from the base at the back of the skull, to the top and the front), that it can be extremely difficult to control both our fantasies and our visceral reactions to them at the same time. When we imagine things, we attach the same emotion to them that we would if the thing were actually happening. This accounts for both an Ni's natural uneasiness about their internal physical state and for the "law of attraction" paradigm. The INFp can imagine both emotionally positive and negative scenarios, and it can be difficult to separate one's feeling right now from the feelings they are experiencing in the scenario. But a strong INFp can keep a positive disposition in his thoughts by reassuring himself that, "The best way to fail is to expect yourself to, and vice versa." I suspect that the things the INTp imagines have little to no emotional charge. While sensing enables man to act very quickly after the fact, Ni enables man to act before the fact, if you will.
Se means acting out the scenarios Ni imagines; for INFps this is imagining social encounters, and for INTps it's imagining something else. I don't totally understand what it could be, honestly. But they do say INTps are the most likely to be socially awkward, so they must be imagining everything BUT people.
**many of you may not agree with this; some people like to think there is something mystical about emotion, and that it is what makes us special**
On Fi: since Fi is introverted emotion, and emotions are evolved patterns of behavior (like physical memories, Si, that have become so internalized that they become biological) whose purpose is to solve problems (i.e. what would we do if we didn't feel fear when we were about to get hit by a bus), this would mean that emotion developed earlier in our evolutionary timeline than Ni, and so Ni can preside over it with enough concentration. Buddhists preach that, during meditation, one must be aware of one's thoughts, but not associate the emotion with the physical feeling of arousal that one finds they associate with a specific memory or piece of mental imagery. They assert that one must not judge one's thoughts, or the thoughts of others, when practicing mindfulness; they must only observe.
It pains me to hear that often people just don't understand why others "space out". Ever since 5th and 6th grade, I remember not understanding why nobody understood what I was doing. When high school came around, I stopped getting sad and started getting mad! Since I was so vocal about it, " Don't wave your fucking hand in front of someone while they're thinking that's rude," people just sort of accepted it. When I'm at parties people understand. "That's stretch, he's just thinking."