As for Si - I was initially confused with my type because I thought that my Ni was strong. The thing that really solidified my primary-Si for me was when I read this blurb describing Si:
"Observe in a subjective way, selecting and relating facts that others would not, and seeing those facts more in terms of impressions and significance than pure fact"
I had confused Si and Ni, and that's what caused my confusion. For me, Sensing isn't all/only about tangible things. It might be that way for Se, I think.
I absorb details and facts from the outer world and process them through sort of a values/feelings filter before I really analyze them. For example, if I'm not sure how to move forward with someone after a conflict, I might remember specific details of my interaction with them...words spoken, facial expressions, etc. I would filter that through my value system and feelings about the situation and use any conclusions derived from that.
Hopefully that makes sense?
I'm still really new to MBTI and I'm only speaking for myself; I don't know if this is how all ISFJs experience Si.
Hopefully it's ok I derailed from the OPs question which was directed at iNtuitives. I do wish I understood N better. Specific examples (like what INTJMom said) are very helpful, if possible.