"The power of its offering..."

Novelist John Steinbeck was asked to write a preface for a book on writing written by Ronald Mirrielees, his story writing teacher at Stanford University. Here is what he wrote:
"The basic rule you gave us was simple and heartbreaking. A story to be effective had to convey something from writer to reader, and the power of its offering was the measure of its excellence. Outside of that, you said, there were no rules. A story could be about anything and could use any means and any technique at all - so long as it was effective. As a subhead to this rule, you maintained that it seemed to be necessary for the writer to know what he wanted to say, in short, what he was talking about. As an exercise, we were to try reducing the meat of a story to one sentence for only then could we know it well enough to enlarge it to three or six or ten thousand words."