Ah ha! Worldbuilding with constraints

I think I've just had a minor epiphany. It's no good trying to pull plots out of a void. Maybe a better way to say that is that trying to choose what does happen when literally anything can happen is paralyzing. A writer needs to create his own universe by defining what is possible and impossible, likely and unlikely. A fantasy or scifi writer does this more literally when they create a fantastical world, but I think all writers must do it to some extent. There are some thing that can never happen in a Hemingway (or whoever) story, because Hemingway's universe has constraints. I need to define my constraints.