More on ser and estar
It sounds like -- thanks to Carlos and Abby -- the answer to my earlier question is that you need to choose between ser or estar from the point of view of the period of time you're talking about, not from the point of view of the present (nor sub specie aeternitatis). I still see some ambiguity, but this helps a lot. Carlos suggests that the choice of ser or estar typically has more to do with subject matter than with any consideration of tense.
Perhaps books are leading us astray by suggesting that ser has anything to do with perpetuity; many books written for English speakers refer to the concept "always" when discussing ser, but it sounds as if native speakers of Spanish and Portuguese actually have a mostly different concept (which I might try to paraphrase as something like "inwardly", "generally", or "normally"). Perhaps the concept of "always" is actually a foreign concept that creates more confusion about tense than is really necessary. Abby, for example, has asked people in Spain about the situation where the population of a city changes over time; they agreed that it was appropriate to use ser when talking about the previous population, which is obviously inconsistent with "always", "permanently", or "inherently" explanations, but perfectly consistent with "generally" (when qualified by time). And "generally" would still work if most of the population of a city went on vacation but then came back, or if a lot of people visited the city for a special event like the Chinese New Year parade or the Pride parade.
There is a book called Counterexamples in Analysis that one of my math teachers admired. (There's also a Counterexamples in Topology.) The point of the book is to challenge your intuition by showing strange mathematical situations that you might not have thought possible, and to highlight the importance of rigorous definitions. Presumably all of this will also improve your mathematical intuition, by way of destroying your confidence in your original intuitions. I wonder if someone ought to write a Counterexamples in Spanish and Portuguese Grammar that presents (for example) weird hypotheticals about ser and estar involving strange sequences of tenses, inaccurate and contrasting beliefs, unanticipated events, and so on, to try to challenge any straightforward rule. (Taking a page from Douglas Adams, we might ask what happens to grammar in the presence of time travel.)
