People often admonish us to learn the lessons of history, but, even if it were easy to discover what the past was really like, it is nearly impossible to discover its lessons  We are supposed, for example, to learn the lessons of World War I  But what are they
And were we ever to discover what they are, it is not clear that we could ever apply them, for we shall never again have a situation just like World War I
It expresses the position the argument as a whole is directed toward discrediting.