Since reading Keirsey, I have been bothered by the seemingly arbitrary division into archetypes. Yes, it seems to work, but why?
After finding descriptions of the different functions, I think I may have an explanation:
All the SP's have Se as the primary information gathering function. This leads to a focus on the here-and-now, and a certain hedonism.
The SJ's have Si as their main information gathering function. This leads to a stronger focus on the past, as every situation is compared to past ones. This will also lead to an appreciation of consistency and tradition.
Now, why aren't the N's divided in a similar manner? Because Ne and Ni gives the same type of output, patterns and connections, so the difference on a NJ and a NP is smaller than the difference between a SJ and a SP. On the other hand, the intuitive functions can give a broader range of outputs, allowing the decision making functions to become more important, so the difference between NF and NT becomes larger than the difference between SF and ST.
Input are welcome, as this idea is not fully developed, and some parts needs to be expanded. And if you know a website with a good descriptions of the functions, please let me know, for some reason they are sparse (or I can't find them, anyway)