ISTJs are most at home with "just the facts, Ma'am." They seem to perform at highest efficiency when employing a step-by-step approach.
Once a new procedure has proven itself (i.e., has been shown "to work,") the ISTJ can be depended upon to carry it through, even at the expense of their own health.
Flexible, pragmatic, and straightforward, you don't beat around the bush but tend to get straight to the point.
You are quick to size up a situation and quick to arrive at a working solution for addressing whatever problems you may encounter.
Mathematics is a system where many INTPs love to play, similarly languages, computer systems--potentially any complex system. Understanding, exploring, mastering, and manipulating systems can overtake the INTP's conscious thought.