So if you were emitting things from a stationary source of air pollution, you needed to get permits from a federal agency in order to do it. And if you were in an area that already had a lot of pollution, which was pretty much everywhere in the country, you weren't supposed to get that permit if what you were doing was increasing the amount of pollution, if you were putting more stuff out than you were putting out before. The problem the agency addressed was, "What's a stationary source of pollution?" Suppose you've got a factory with ten smoke stacks. Now those ten smoke stacks are producing at a given point in time a certain amount of pollution. Let's just assume that the agency is okay with the factory producing that amount of pollution. The problem is, you don't want to get more pollution out. Well, the factory looks at its ten smoke stacks and says, "Well, you know, eight of those smoke stacks are working pretty well. Two of them really stink. Two of those smoke stacks are just producing way more pollution. We can actually do better if we shut down two of our smoke stacks, move all of our production to the other eight. The total amount of stuff coming out of our factory will be less once we do that. But smoke stacks six, seven, and nine will have slightly more stuff coming out of them at the end. That's all offset by the fact that we've shut down the two lousy smoke stacks, but we'd like to get a permit to allow us to do this.” If a stationary source means every single opening out of which pollution comes, they can't do it because then smoke stacks six, seven, and nine are going to be higher after this plan than before. You can't give them a permit for it. But if you count the stationary source as the entire plant, then you're golden because the plant as a whole is emitting less after the plan than before. The Environmental Protection Agency went back and forth several times through several different presidential administrations about whether it thought it could interpret the statutory term "stationary source" to mean the whole factory or whether it had to be each individual smoke stack. They finally settled on, "Let's treat the factory as one source. We're going to draw an imaginary bubble over the entire factory and that's going to count as a single stationary source." That was challenged in court and that case the Supreme Court described this two step inquiry. It said, "First we're going to ask, does the statute clearly resolve this? As we look at the term 'stationary source' is it obvious to us that it means either the whole factory or an individual smoke stack or anything? Nah, it doesn't. No answer leaps forth. So that leads us to the second step of the inquiry. Is the agency's result here reasonable? Is it so far off the map that we're going to reverse ... Nah, it's fine." Now there was actually a great deal of precedent for that kind of approach before 1984. In a certain class of legal disputes where the agency wasn't just interpreting the law abstractly in the air but was applying a particular legal term to a set of facts. Occasionally, before 1984, not frequently but occasionally courts would treat pure or abstract questions of law as also appropriate for deference after a multi factor, all things considered, "We think this is one of the cases where there is good reason to give the agency view some weight."