INTP - Top down.
ISTP - Bottom up.
Depends upon when you come to interpret rules does it feel more natural to start with what you would consider the rules as a whole have as an intent or what the rules actually say individually?
Definitely possible to confuse the two though. My ISTP friend mirrors me quite a bit... or I mirror him...not sure who started that one. Anyway in general he's more specific than I and locates his thinking on specific instances where as I locate on trends. We tend to cross where I note specific abnormalities in patterns I've observed or when he's doing complex thinking where his original case examples aren't obvious until after the subsequent four hour argument.