But is that really a premise? By some definitions, maybe not. Another definition, more steeped in the study of logic, goes like this: Premise is a statement assumed to be true and used to draw a conclusion. It's a presupposition, assumption, or postulate. More specifically applied to story, it's a statement of what happens to your characters as a result of the plot, which your story sets out to illustrate.
Which is an interesting point of view. Math major though I may have been, my definition of story premise has always been more in the first camp, the what-if question. So what is premise? A question, or a statement? Does a premise include the ending, or doesn't it? By the statement definition, the premise of Romeo and Juliet might be "Stubborn feuds or prejudices lead to tragedy." The statement contains "what happens to your characters"--tragedy--and the what-if question does not. If it's correct to say that the statement is the story's premise, then we can't know a story's premise when we first pick it up to read it. We can only know the premise when we know the end. I'm going to have to chew on that a little more before I buy it. If I buy it. Perhaps both kinds of premise are useful: the question as the public one, the hook; and the statement as a private one, a guide for the writer.
Both viewpoints agree that a story has one premise. But it can have multiple themes. Speaking of which: doesn't that second view of premise, in the preceding paragraph, sound an awful lot like theme? "Stubborn feuds or prejudices lead to tragedy"? If that's not theme, then what's a theme? We often hear statements such as "This book explores themes of love and loss, friendship and courage, sacrifice and honor..." and I say No. These aren't themes. These are subjects. Themes say something about a novel's subjects, and we don't have control of our work if we don't know what that "about" is. Let's go back to Romeo and Juliet. Does it explore themes of passion, labeling or stereotyping, friends and enemies, and communication? I think it explores those subjects, and makes theme statements about them.
Theme: Quick passions lead to hasty decisions.
Theme: Our enemies may not be our enemies.
Theme: Miscommunication leads to disaster.
What do you think? What's a subject? Theme? Premise? Does your brain hurt as much as mine does? :)