Introverted iNtuition (Ni) is one of the eight cognitive functions outlined by Carl Jung. Intuition (N) is an outlook that focuses on the “filling in” of experience with [mental] “constructs” such as concepts, hypotheses, or theories, which all involve “larger contexts” or meanings behind things and [non-physical] “patterns”. Even physical or visible things, like in comparing one thing to something separate, but has some sort of inferred similarity. Focusing on a property to compare, like its shape; the person has turned into an “idea” (as in "the idea of" things). This is what “could” or “won’t” be. An "introverted" attitude (i) indicates that this outlook generally draws from the individual; or the inner world of the "subject" (individual ego), instead of directly from the outer world of the environment.
So introverted iNtuition (Ni) can be described as "awareness of hypothetical reality inferred from the individual", where one turns inward to “fill in” experience of objects (often existing connections or patterns) with mental/ideational constructs such as "images" or "visions" that come up from the unconscious (including through dreams), or just plain "hunches" that have no objective evidence. Every type does this, but for types who prefer Ni, it will be their main form of information-gathering, rather than intuitive data totally from the environment (Ne), or sensory "facts" (S).
Its preference by a personality type is indicated by the "_N_J" code, and the types holding it as "dominant" are INTJ and INFJ, while for ENTJ and ENFJ, it is "auxiliary". (For SP types, it is tertiary or inferior, and may also develop at some point in life).
Since it is "subjective", starting from an individual impression that is then used to filter the external world, and also "abstract", meaning it deals with data that is not tangible and accessed by the physical senses, this has been the hardest function to understand or explain, even by those who prefer it. Part of the confusion is that it can so easily be confused with other functions, such as its extraverted counterpart (Ne), and even the other introverted functions, of Si, Fi and Ti; all of which similarly draw upon "deeper" data from within.
Comparison with other functions
To begin understanding the difference of Ni from other perception functions, especially Ne, it's useful to determine what really determines the difference between the perceptions of Sensing and iNtuition, and especially the "attitudes" that distinguish Ne and Ni. First, we can start with the fact that Jung identified iNtuition with time; "where an object is coming from and where it is heading" (which are invisible implications or inferences of the object).
By comparison, the Sensory data, by which we become directly aware of the object, can be seen as occurring through space. All sensory perception is spatial. We see or hear waves that come to us through space, smell particles that float to us through space, and touch and taste things we reach out for through space. This occurs in time as well, but all the perceived objects are experienced through space. Time is where occurs the “possibilities” of where they can go.
So now, to factor in the attitudes, extraversion deals in the “environment”, while introversion is about the "individual". Both space and time consist of linear “dimensions”, of biploar “directions”, by which every conscious entity immersed in it divides reality. (Just like the four functions and two attitudes themselves are divisions of reality). Space has three (randomly accessible), and time has one, which is one way.
So Se is basically what you experience in the immediate environment, as you look out into any of the three dimensions of space. Si is the same spatial data, but stored individually in memory.
Ne then involves what you experience when following the chain of occurrences when looking through the dimension of time. Its inferences occur along this time line (the "environment"). Hence, what “could” happen. Also, following past patterns, and continuing their trajectories to get a sense of what will happen. (Of course, things can change, and so Ne remains “open”. Some of the confusion likely stems from determining “the outer world” for N, since iNtuition is all technically “internal”, and imagines possibilities for things on the external world).
So then Ni also looks at the dimension of time, but its inferences do not come from the timeline, but rather from the individual, which is the unconscious. This is the domain of the “archetypal” (images that are collective, and not tied directly to our external experience, hence, “timeless"; meaning pervasive through time; not on our individual timeline of experience).
Lenore Thomson had defined Ni in her book (Personality Type, an Owner's Manual p223) as “liberate our sense impressions from their larger context, thereby creating more options for perception itself", which might be hard for non-Ni types to really grasp. (Where Ne was "unify our sense impressions with their larger context, thereby creating new options for meaning and response"). The timeline idea explains it. The “larger context” is what occurs in the time dimension, but the “unconscious impressions” are from outside of the time dimension, and so you can get more kinds of interpretations than what were available in the temporal environment. The example given there is raising the question in one’s mind of the possible reasons a suntan is valued by people today, when the original circumstances that gave it its meaning have changed. Again, we see the time element of this, and the pondering steps outside of this timeline to raise the question of why it’s still considered attractive.
Likewise, “Perspectives” is the single word nickname the online typology teachers called "Personality Hacker" gives to Ni, and they describe it as “not married to its own perspective”, and “watching your mind form patterns”, and eventually, over time, you’re going to get the ‘pattern of the pattern'” [hence, “meta-awareness”], and so when listening to another person they can shift out of their own perspective and into the other person’s perspective and get a sense of what’s going on with them, and be able to guess “I bet this is the pattern created in the other person’s mind”, so it looks like reading their mind. https://youtu.be/RhYz0Bz_3mk These would be the “internal connections” corresponding to Ne’s “external connections”).
Further Comparison with extraverted iNtuition
Both Ne and Ni are associated with "connections", and it often becomes hard to tell which is which from the definitions. But Ne's connections between different things would be like an NTP type theorist connecting together different personality typing systems according to corresponding elements. These elements (such as factors and temperaments) would be the external objects being focused on. The connections between them are likewise external (such as different systems having analogues to "extroversion").
Ni's connections on the other hand are deeper and harder to pin down, but would involve elements such as "meanings" that underlie the surface parts. In short, inasmuch as both forms of iNtuition deal with "inferring" (even though some theorists have associated it with Ne only), Ne is inferring a pattern from the object (often in comparison of another external pattern), and Ni is inferring from an internalized (subjective) impression, used to find out "what's left out" of an external pattern.
When we explore alternative possibilities in the environment, we are engaging in external intuition. When we look at on one of them, and fill in its "possibilities" from within, we are internalizing the intuition process. (It should at this juncture be pointed out, that "possibilities" is often associated with Ne only, but really, all four perception attitudes deal in possibilities in different ways.
Se looks at possibilities as what can be exploited based on what is. Instead of simply what's "possible", a more accurate term would be what's "doable". The N attitudes are about what is not necessarily what's there, and you either gauge this from other objects, or from within. Si, then looks at possibilities, according to what has been exploited in previous experience or common knowledge, rather than emergent data).
When we make connections between objects based on properties of the objects, it is external intuition, for the aspects of the objects we are connecting are external to us. When we connect things using images from the unconscious, it is internal; for our own unconscious is of course internal.
So Ne is inferring a pattern from the object (i.e. What this means as a pattern), and Ni is inferring from an internalized (subjective) pattern (i.e. What's been left out of a [given] pattern, existing now, as an internal intuition). Lenore had said “For INJ’s, the patterns aren’t ‘out there’ in the world, waiting to be discovered. They’re part of us [i.e. internal]—the way we make sense of the riot of information and energy impinging on our systems.” (p.225).
The "subject" in the case of Ni, is the unconscious itself. Hence, NTJ's will often be more likely to tend to be skeptical about the ideas NP's toss around. (With NT in general being characterized as "skeptical"; the J variant will be, even moreso). The NP will throw out an idea (like something involving temperament theory), and the NTJ will seem to start with some preconceived bias against it, and want to pick holes in it. For some reason, they just "know" those ideas are wrong, and something else is likely right. (If another NTP is skeptical, it's usually because they've simply adopted another logical framework they give their allegiance to. But both will give their technical, conceptual arguments for why they believe something else is true).
So just as you can experience a current event in the environment, just for what it is, which is Se; or abstract a new meaning from it through other patterns in the environment, which would be Ne; you can also look back at an event, just for what it was (individually through memory), which is Si, or abstract significance from it in the form of things such as these impressions (individually from the unconscious), which would be Ni. While Ne started from an external focal point, and then branched out multiple possibilities from it, Ni has been described as the opposite; starting out with multiple objects, and then converging the possibilities to one [internal, known only to you] focal point, which might be a likely [future] outcome. (Hence, the orientation of iNtuition can be determined by where this focal point lies).
With perception basically answering "yes/no" (where judgment tells us if it's "right or wrong")
Ne yes/no (pattern of where it "could be" heading or not) determined by the objects involved themselves
Ni yes/no (pattern of where it "could be" heading or not) determined by internal unconscious
(Where Se is yes/no [what "is" or not] determined by whatever is physically before you, and Si is yes/no [what "is" or not] determined by whatever you remember physically experiencing).
Both Ne and Ni "fill in" things from a "big picture", but Ne lets the object or entire environment fill in its own big picture, where Ni looks within to access a wholly separate big picture, that others likely can't even be made to see.
Ne will tend to say "yes" to everything, because looking at an object, you can imagine anything for it. It's basically the judgment function or tandem-mate Si that would have to indicate an outcome is not tenable or viable.
Ni will say "yes" or "no" (and more often says "no", especially to things Ne takes for granted) based on a hunch, gut instinct, or image that just comes up. Then, Je will be used to verify the "yes" as "right", or "no" as" wrong".
One NJ's Ni might say "no" to something, while anothers' says "yes". Both then use Je to verify their perceptions. Either may appear to be just "pulling things out of the air" when it comes to either an idea, or an objection to something, but it's Ni, behind the scenes just perceiving it as tenable or untenable (able or not able to be held), for reasons that might not even be conscious.
Any image or interpretive thought that comes up (that isn't a past concrete experience), that locks on to a "yes" or especially "no" to a possibility, regardless of what the involved object itself would allow, is likely Ni.
It's "automatic-ness" is similar to Se, hence both functions in tandem fitting "Realizing Awareness"
An example of introverted iNtuition
One INFJ () described "images" that come up, and that "life feels like an ongoing tidal wave of intangible, unverifiable information". These images are for all of people, coming up all the time. Other types basically tune them out, unless they happen to fit some external pattern they may be thinking of. So it's easy to never even think of them as the products of one of these "functions", and hence why it is so hard for most to really understand what Ni is.
For her, they'll come up like that, and naturally, they'll also come up when deliberately called to inform some data (like an existing pattern you're presented with), and hence, Ni types also described as "search[ing] within". Si/Ne preferring types may "look within" to memory, for a fitting pattern, but that's really Si internalizing an Ne external pattern. It's not the same as what she's describing that already comes up, directly from within (seeming out of nowhere, not pulled from somewhere else), and not conscious memory.
In contrast, for Ne, one compares and then begins finding several connections, and can then begin sharing them (like in "brainstorming"). Other people can see the connections. They may not agree with them, or still have things that don't make sense to them, but they can clearly see the parallels being made. Because the things being compared are all external "objects", already existing in the environment. They are not things anyone has conjured up from within. So the connections of Ne (which are abstract, and thus "unconscious" in a way, they have to be made, intangibly) are all there, waiting to be shown to others.
Se/Ni tandem vs Ne/Si
Ne’s patterns “stored in memory” by which it actually does its looking down through the dimension of time is precisely what makes it work with its opposite tandem mate, Si. Hence, both are associated in the new “Intentional Styles” model of Linda Berens and Chris Montoya with “Inquiring”; which is basically going mentally through (e) time (N) to access previous (i) spatial experience (S). Se’s immediate (e) space (S) orientation then works with Ni’s immediate “outside (i) the [timelike(N)] pattern” awareness, and hence are called “Realizing”.
So both Si and Ne end up relying more on memory, and thus also end up "inquiring" (asking questions) in order to either match to what's known, or look for other possibilities. Se on the other hand, simply takes whatever's there in the material world. Ni takes whatever comes up from the unconscious. Neither try to compare it with anything else (it will be the judgment function that does that). So they both directly "realize" things.
Se=environmental material awareness (direct sensations)
Si=individually referenced material awareness (though memorized sensations)
Ne=environmentally referenced hypothetical awareness (external hypotheses, often memorized)
Ni=individually conjured hypothetical awareness (images that come up directly from the unconscious).