Disclaimer: these are again my musings. Since I haven't thought as much about this as about the Fe-Ti and Fi-Te divide, and/or because I'm not irrational, these thoughts are more half-baked.
Obvious remark: everyone thinks of "present reality", of the future, of dreams, and tries to improve reality and reach their dreams - to some extent.
- both Ni and Si refer to sequential cause and effect correlations. In one area of quantum theory, that is precisely one definition of the time flow.
- Si and Se are close to reality; in that they are close to how our five senses perceive immediate reality.
- Ni and Ne are further from reality, for the same reason.
- Both Ni and Si are dynamic.
So, one way to define Ni and Si is:
Si: dynamic "time flow" closest to the senses, that is, to present reality
Ni: dynamic "time flow" far from the senses, from present reality.
On Ne and Si:
Si is the perception of "present", "real", "close to the senses", "here-and-now" dynamic time flow.
Ne are "static snapshots" within, and escaping from, this here-and-now flow: how to shape the present reality into something closer to our wishes?
On Ni and Se:
Ni is the perception of "non-present" time flow, not close to the senses: the future, the past, or some imaginary world; dreams.
Se are "static snapshots" of reality, "bringing us down" from the Ni flow: the realization that Ni isn't "real" yet, and that it's still necessary to get there.
The Ne-Si perception:
The "down to earth" Si dynamic flow, in its pure form, is a "relaxed", vegetative state: the total surrender to immediate sensorial perceptions.
The Ne "snapshots" within this flow reveal how this dynamic flow can be brought into improved situations.
Essentially: the Si flow is good; Ne snapshots show how to make it better.
If you are caught only in the Si flow: you'll never make it better.
If you are caught only in the Ne snapshots: you'll never come "down to earth" to enjoy the peace of a dynamic Si flow.
The Se-Ni perception:
The "dreamy" Ni dynamic flow, in its pure form, is a "relaxed", vegetative state: the total surrender to inner, non-sensorial, non-immediate perceptions.
The Se "snapshots" within this flow reveal that the Ni flow is not real, and at the same time, that it must still be achieved.
Essentially: the Ni flow is good, Se snapshots show that it's not real.
If you are caught only in the Ni flow: you'll never do anything to make it real.
If you are caught only in the Se snapshots: you'll never know what is it that can make you reach the peaceful state of relaxed flow.
In other words:
Ne-Si: a "down to earth" state of relaxed enjoyment of the present, "real" time flow, "interrupted" by Ne "visionary" realizations that it can be improved.
Se-Ni: a "visionary", "dreamy" state of relaxed enjoyment of the non-present, "dreamy" time flow, "interrupted" by Se "down to earth" realizations that it's not real and has to be yet achieved.
Si: down-to-earth, Ne makes you "take off" until calming down back to Si.
Ni: Visionary, Se makes you "crash-down" and want to "climb back" to Ni.
(where "climb back" is actually only an imaginary situation, since you were never "in" Ni in the first place, not in "reality").
Ne-Si: a contented enjoyment of the present-time flow, interrupted by "jumps" that make you thing of something to improve it.
Se-Ni: a focus on a "desired reality", a "dream", or "future", with "jumps" reaching out at it, making you want to actually get there.
How Ne-Si sees Se-Ni: "never contented", "not down to earth", "too ambitious", "more concerned with a future that may never come than with making the best of the present".
How Se-Ni sees Ne-Si: "no vision", "no ambition", "too contented with the present", "so focused on the present as not to think about the possibilities of the future".
Ne-Si: the enjoyment of the present with the insights on how to improve it make it "reasonable"
Se-Ni: the lack of contentment with the present, with eyes focused on a not yet achieved "dream" or "future", and the awareness that it's not real yet, make it "resolute".
The downfall of Si without Ne: contentment with the Si present flow with no initiative to improve it, to make it more enjoyable, more interesting, amusing, better
The downfall of Ne without Si: not being able to "sit back and enjoy" the present flow.
The downfall of Ni without Se: the "dreams" and the "future" will ever remain unachieved, perhaps without even realizing it
The downfall of Se without Ni: not being able to actually see what is it that needs to be achieved, and how far away from it you are.