Ever have one of those moments when you've been trying to figure something out for a long-ish time (measured in years in this case), and it's one of those "imponderables" where there are a million people with a million opinions?
One of those tough nuts where philosophers are the main people discussing it, and mostly they're just talking about how people feel about the question and what the question means and what they want the answer to be, but you need an actual specific real-world answer? Where there's no consensus anywhere and you've had a hundred ideas that, when you examined them closely, weren't quite right.... and then someone tells you something and you instantly know it's the correct answer, and you probably knew it already but not consciously, and it turned out to be simple all along?
Yeah. I had one of those yesterday. I picked up what has to be the fiftieth book or so on the topic that I've read. I didn't expect to find the actual answer in a book; I expected the discussion in the book to incrementally stir the ideas around in my head so I had a more complete perspective to think about it, at best. But, defying all expectations, there it was.
Kudos to Lisa Feldman Barrett, for finally writing it down.
The big question was "what requirements do emotions fulfill and what fundamental biological purposes do they serve?"