I've said this on like five different threads, but I figured I'd actually make a full thread about it.
I think that Ni relates to Keats' idea of Negative Capability. Negative Capability is defined as that state "when man is capable of being in uncertainties, Mysteries, doubts without any irritable reaching after fact & reason." Ni seems to be related to this capacity to work productively in uncertainty. Just as Se lives in a very certain, defined world (external statics of objects involves the least mental manipulation of any of the functions, it is the most directly attached to the external world, it is pure sense perception taken as fact), and this produces many if not most of the qualities we associate with Se (decisiveness, confidence, resoluteness), Ni lives in a very uncertain, undefined territory (internal dynamics of fields is the most mental function, the function furthest from the objective world; also, a great metaphor can be drawn to "undefined" functions in mathematics or to Heisenberg uncertainty in physics), and as such takes to situations of uncertainty where there is a lack of facts or certainty like a fish to water.
The obvious example is poetry, much of which is written by Ni-egos. Poetry (especially American poetry, which is heavily Emersonian) is all about throwing away the old forms and disregarding old certainties and reevaluating everything through one's self. When Whitman talks about "creeds and schools in abeyance" he is using an external metaphor for an internal process of not believing in anything that we consider to be certain, like religion. Similarly, when Rimbaud talks about becoming a "seer" by a "rational derangement of the senses," he is also using an external metonymy for an internal process; to stop believing in that which we take for granted is like not receiving information from any of our senses. And the thing is, poets, like Keats and like Shakespeare, work productively in the uncertainty that results from the disavowal of basic conceptual assumptions. They make their own discoveries with the aid of intuition, and above all they describe things even as they recognize that what they describe is contradictory. But in the metaphorical act of "speaking" the contradictory world, they in some ways create order out of chaos. They make valuable observations by trying to describe the world without any of the old conceptual models, without any facts or certainties.
There's also a good comparison to contemporary physics (more a gamma NT activity than a beta NF one, in general); we discovered that the old conceptual models were rubbish (that was largely Einstein's doing, I suppose), and now we're trying to build new ones, to conceive of a world made of strings rather than particles or some craziness like that.
This also relates to the principle weakness of Ni-superego (especially in delta STs): a lack of Negative Capability. An inability to work productively in situations wherein there is uncertainty and and mystery and doubt and a lack of facts. So LSEs can't really operate if you don't give them any facts to work with. They can deal with some blurring of the boundary lines, but when you tell them that the boundary lines we've assumed all along actually don't exist, and that none of the "facts" apply, they can't work effectively. This is a principle source of conflict between LSEs and IEIs. The IEI says that life is inherently uncertain, mysterious, and that's okay. The IEI has to believe this, because otherwise, life is boring. The LSE, on the other hand, says that life is full of indubitable or at least obvious facts, and we have to deal with those facts. The LSE has to believe this, because otherwise, life is nonsensical, and you can't make any progress.
In summary, there are some situations, including, possibly, the state of socionics in the West, in which there are no clear-cut facts, no basis or foundation of data that is commonly agreed upon as true, no certainty. True tragedy is one of these situations. It is in these situations that Ni-egos shine, and Ni-polr types have the hardest time. Ni is the function most suited to uncertainty, lack of clarity. Thus Ni is an intellectual/spiritual/psychological pioneer just as Se is a physical/practical pioneer. Ni makes strangely accurate guesses by observing the situation with minimal givens. Ni as a mode of thought works productively in uncertainty. It is "I don't know the facts, but I have a guess." In situations when the facts are truly unavailable/nonexistent (rather than the Ni-ego was just too lazy to go find the facts), Ni shines.
Some related thoughts. (feel free to stop reading here)
I've been generally considering this in terms of IEIs, beta Ni, which is paired with another very abstract function, Fe. But Te is the second most concrete function (after Se). So this produces two possible hypotheses about ILIs and gamma Ni. Either a) gamma Ni manifests this quality to a lesser degree than beta Ni, because it is "grounded" as it were by Te, or b) there is a sense in which Fe represents a sort of certainty just as Te does, and ILIs/gamma Ni has a similar sort of uncertainty, just related to a lack of Fe-certainty (for which I do not have a name) rather than a lack of Te-certainty (which we can call "facts" or "data"). Maybe part of the clue to hypothesis b is in Falstaff, who, Harold Bloom says, has perfect faith in language, where Hamlet has no faith in language (that is, he's in doubts, mysteries and uncertainties, even about the language of which his thoughts about doubts, mysteries, and uncertainties is comprised). Since Falstaff is clearly an Fe-leading type, maybe he in some way exemplifies the "Fe-certainty" I'm questing after.
How does this all relate to Ne? Isn't Ne also capable of being in mysteries and uncertainties? But "irritable reaching after fact" is pretty much a perfect xEI description of Te-seeking/valuing. What is the difference between Ne dealing with uncertainty and Ni dealing with uncertainty? Maybe both of the intuitive functions are good with this kind of stuff? I don't really have answers here.
How does this differ from or relate to Descartes' Radical Doubt, which to me is a much more LII sort of thing. How does it relate to rationalism or the lack thereof?
I'd also love a delta perspective on this, especially LSE. It must seem like just the opposite to LSEs, like IEIs are creating confusion where there is none or something (whereas I find that LSEs are clinging to an illusion of certainty where there is none). I remember Ryu made a blog post once that very much influenced by view of Te, wherein he argued that delta Te most purely seeks the truth, and from an aspectonics POV, this makes sense to me: delta Te is the closest thing to the objective world that will submit to mental manipulation. It is, in some ways, the perfect marriage of objective and subjective, or the limit of objectivity that will still submit to be taken subjectively, that can still be examined as a mental "object" or fact. But I can't quite get my mind around how deltas view these facts as essential and primary to nature, and yet my theories on life tell me that facts are just as essential and natural as uncertainty (see: wave-particle duality--or in this case wave-particle conflictality, lol), just somehow from a different angle. If I'm going to become magic and type transcendent like Shakespeare (or at least do a good job of making Te-valuing characters as more than stereotypes), I really want/need to figure this out.