Law as written: All demonic influences are to be removed as quickly and thoroughly as is possible.
Letter: Utterly destroy anything that has been touched by a demon.
Spirit: Exorcise demons as efficiently and with as little death as is possible.
Reason: Protect the people.
Let's say we have a situation where there is a demon rampaging around town, possessing people, etc. By the letter of the law, one should utterly destroy the town. By the spirit or the reason, one should evacuate the town and fight the demon down. There are probably some other things you could do; I'm hitting extremes; just go with it.
Let's say a demon is repeatedly possessing the same person. By the letter, kill the person. By the spirit, exorcise the demon and watch and protect the person. By reason, set a priest to watch over the person, one who can perform exorcisms. The priest need not follow the person everywhere, it is just supposed to be the priest's main duty, so someone is always on deck. If the demon gives up long enough, then the priest can go on to something else. But there should be someone present who can do this, even if it isn't the primary job.
Let's say there's a good demon. Go with me on this. By the letter of the law, kill the demon anyway. It doesn't matter whether harm is being done, a demon is a demon. By spirit, get the demon out of town, one way or another. If following the spirit well, then this can even be done without bloodshed. Following the reason: Let the demon be. This demon is not dangerous.
People will talk about how a law should be upheld because it is the law. In situations where one requires a knee-jerk or immediate response, this is probably the best one can do. But if one ignores the reason of the law, that can be worse than ignoring the spirit. At least people who violate the spirit in favor of the letter are easier to expose. For people who follow the spirit instead of the reason, there's not even anything on the books.
This post written under the influence of painkillers (see previous post).