I confess I haven't read a single word of this thread, but that won't stop me from throwing in my 2 cents:
OK, so I'm pretty sure both Si and Ni are about anticipating things before they happen, whereas Ne and Se are about reacting to things while they happen. So both Si and Ni draw on the past, and both Si and Ni predict the future - but they don't share the same focus.
- Introverted Sensation is about physical anticipation - you can "feel" the physical outcome of something before actually experiencing it, because you're drawing from and reliving a rich database of past sensations. You feel sick when looking at a piece of food because the food looks or smells like a bad experience you've had in the past. You haven't actually tasted the food, and other people might think that there's nothing wrong with the food, but you're still convinced that it will be bad for you.
- Introverted Intuition is about contextual anticipation - you can detach yourself from the meaning of things and think in terms of symbols or archetypes being applied to different contexts. By doing this, you just "know" the outcome of something, because you've subconsciously picked up on patterns and symbols that point to it as an inevitability. You have yet to experience this, because it hasn't happened yet - but still you know, with almost absolute certainty, that it will.
This is probably only a small part of the difference between the two, and I still can't wrap my head around Ni enough to give a genuine description of how its experienced, but it just happened to be something I was thinking about today and I felt like sharing.