Si: [...]
Se: [...]
Ne: [...]
Ni: [...]
Discuss...
Good idea. I'll contribute with a comparison of how N and S work. It's not really about sandwiches, but maybe it will stimulate some thoughts for others.
This is just my own interpretation, of course. I wrote this up on the fly; excuse the typos or poorly worded parts.
First off, a quick comparison of N&S vs F&T:
--N and S are the "irrational," perceptive, creative functions.
--F and T are the "rational," judging, analytical functions. I'll just add: Some people have difficulty thinking of F as an analytical function. But as F-users mature, they can become very good at parsing and analyzing emotions, ethical issues, where and how much empathy to bestow on various people or issues. They are often good at counseling and can help people parse issues like love, emotional bonds, grief, betrayal, etc.
Now I'll talk about N vs S specifically:
--N is associative, in the sense of Freudian "free association." It looks for far-flung connections between disparate things, and it's good at detecting or inventing flows, conspiracies, plots, etc.
--S is causal, in the sense of "cause-and-effect." S looks for immediate connections (X leads to Y), and it creates narratives, rituals, and automated procedures.
--As you can see, both N and S are creative and good at constructing something from nothing. But creative processes are non-analytical, and they can lead you astray...
Now I'll talk about Ni versus Ne specifically:
--Ni is associative, but in an internal sense. It likes seizing upon interesting thoughts and ideas that seem to have some internal similarity and stringing them together. It plays with favorite ideas and associations over time, eventually creating vast spiderwebs of favorite "idea complexes." Hence, you get the Ni stereotype as diabolical masterminds. They massage a favorite "idea complex" and create plans and backup plans. This is a hallmark of "spiderweb thinking": Spiderwebs have lots of entrances and exits. Ni-users are the most likely types to come up with something like universal "forces" or "flows" or "marches of history." Big thinkers by nature, they'll string together vast associative spiderwebs that encompass all of history, trying to find some common thread or theme. But again, being non-analytical, this kind of thinking can go astray. Ni-users can create whole religions out of thin air, simply by "shoehorning" all kinds of crazy associations into a favorite "spiderweb." Hence the need for an extroverted second function to keep them grounded.
--Ne is similarly associative, but in an external sense. It plays with the raw material provided by the world around it, stringing it all together into plots, conspiracies, intimations of love or hate, etc. Ne sees everything in the world around it as connected (in an associative sense), so it's easy for Ne-users to be super-sensitive to small changes in a relationship or social setting; Ne-users are the most likely type to be psychics and readers of other people, and actually be pretty good at it. But like Ni-users, they can overdo it. Ne is non-analytical, and if over-indulged it can create secret societies, conspiracies, plots, etc. out of thin air. Ne-users need to consult their internal judging function to help separate the wheat from the chaff.
Now Si versus Se
--S is causal, but in an internal sense. It's a narrative-creating function: "X caused Y, which in turn created grounds for Z to arise." Spot the narrative, and you own the world. S in general is the most adept function at maneuvering in the world because it's always on the look-out for causal connections and memorizing them as narratives. From this, you get rites and rituals; Si is fertile grounds for the creation of automated routines that serve a predictable function or purpose. Si-users are traditionally good matches for the military, police, the bureaucracy, etc. Their knack for narratives and rituals makes them excellent at spotting the workings of things in the world in front of them and coming up with ways to replicate success. As I said above: Spot the narrative, and you own the world. In a religious sense, there also exists a "warding-off" function to Si: Narratives and rituals ward off chaos. The gods like order, predictability, and precision. Hence the need for rituals to be repetitious and performed exactly the same every time.
--Se is similarly causal, but in an external sense. It plays with the raw material provided by the world around it, creating personal narratives on the fly. "X exists; if I take X and do Y to it, I should get result Z." As actors and initiative-takers, Se-users are constantly "taking it to the next level." Things need to be tested and prodded and poked in order to extract their narrative from them. And once you spot the narrative of a thing, you own that thing. Again, narrative-creating is also about rituals and rites. In a religious sense, there exists a "summoning" function to Se: Spot the narrative of a thing, and you incorporate its spirit and gain its strength. It's a form of totemism: Compete with your enemy, beat him, and then "summon" his spirit to make yourself stronger. Hence, a natural competitive aspect to Se. The gods reward victors and heroes; what doesn't kill you makes you stronger. Spot your enemy's narrative, and you own him. And the gods reward you accordingly.