I've noticed a possible new dichotomy that applies to each IM Element. I call it playful vs. constructive.
I wonder if it's related to irrational vs. rational. Perhaps it may also be similar to what some people consider to be - vs. + (?)
However, I suspect that it may be partially or completely independent from those things. It may also apply more to what people say about the IM elements than what they really are, but I still think it's worth considering.
Here's how it works:
I've noticed that some descriptions of Si emphasize the playful, hedonistic quality: doing things for pleasure, liking to eat, eating dessert, relaxing, doing stuff that maybe isn't so good for you like drinking and smoking, etc.
And then sometimes people describe Si in terms of healthy behaviors: doing what's good for you, taking care of your body, which means not eating too much, not eating dessert, exercising a lot, and not engaging in any bad habits like drinking and smoking.
So these things are exact the opposite.
A similar thing can be seen with Ni. Sometimes people talk about Ni in terms of being really great with time, estimating how long things will take, focusing on what's likely to come up, being timely, doing things at the right time.
And sometimes people talk about Ni in terms of daydreaming, ignoring what's happening, being in one's own world, dwelling in the imagination, being disconnected from worldly schedules.
....Again, these things are the complete opposite.
Actually, it seems that each IM Element (at least as commonly discussed and understood) can be divided into "playful" and "constructive" behaviors which are complete opposites from each other.