Si is almost how we perceive inferior Se to be.
Si is not as aware of what is currently happening, it is an introverted process - it is focused on the self and it's own subjective impression of what is happening.
This is where Si ties in with the whole "memory" thing - because Si is focused on it's own personal experience of sensing, rather than actually interacting with the outer world itself - it makes sense, therefore, that other similar personal experiences would spring to mind for the Si user (whatup Ne?).
This is where the foundations of the whole "traditional" stereotype stem from - due to the Si users preference for focusing inward on it's own subjective impressions of a situation - and the similar situations that manifest in the Si users brain that resemble the current subjective impressions - it makes sense the Si user would find the most comfort in, and enjoy, doing things that aren't a far cry from what they have already enjoyed.
This kind of nostalgia is what has people rattling off about Si users being stuck in the past, narrow-minded, short-sighted.
The truth is, that this isn't actually the case - they aren't immediately opposed to anything and everything new (it's not a judging function) - they simply prefer some kind of grounding, some kind of familiarity.
If they can find something to connect how they are experiencing a new situation, to a subjective impression from the past they have enjoyed - they will be perfectly fine, they'll have found the common ground. The missing link.
Therefore the Si dominant is excellent at adapting to new situations, implementing new systems, "going with the flow" etc (fuckin' bang, there goes the Si stereotypes) - as long as they have that connection - because their subjective impressions of previous experiences will guide them with regards to how things "should" be.
I believe Si is equally as abstract as Ni, they're essentially guided by their own subjective impressions of things.
How is this not abstract.
To compare to Se, in the shittiest way possible - Se is concerned with what is currently happening outside of the person.
Se is energised not by what is within the person, but the outer world - it wants to engage with it's surroundings and experience the present moment, the literal, physical experience itself is what the Se dominant is after.
Se is therefore seeking to enjoy what is happening. Satisfaction.
If this is someones preferred function, it would make sense that they would develop a reliance on the outer world - a reliance on their 5 senses - they are concerned with what they can see, hear, taste, smell and touch.
That is reality to them. That's what is real. Anything that doesn't fit inside the scope of those 5 senses would, I imagine, be unreliable, unverifiable.. unreal.
This is probably why my ESTP friend wants to literally see me play the Sweet Child o' Mine solo - he won't believe I can do it until he sees it, hears it, experiences it himself. Then it will be real (and satisfy his Se cravings).
He's also been told by other people that I can do it - others have seen it - he still thinks it's bullshit and needs to see it himself.
Anyway, that's kinda my understanding of Si, it's a combination of a various points of view and my own.
Not really on-topic.
I have tertiary Si (or is it Se? Where's the proof the tertiary function is the same direction as the dominant function?).. so, maybe I don't know what I'm talking about.
I think it's easy to see how Si perfectly matches peoples understandings of Ni.
I mean psychic voodoo shit like deja vu can easily slip into Si.
Same with premonitions and foresights - you can just see how Si could be behind all that kind of thing.
Makes sense to me.