I need hardly remind this audience that one of the characteristics of our field is the large number of old and quite elemental questions—elemental but by no means elementary—for which we have no compelling answers. I don't mean that we have no answers to these questions. On the contrary, we often have a rich variety of conflicting answers. But no answer compels acceptance in the same way as a proof of a theorem in mathematics, or a very nice fit between a hypothesis and a satisfactory set of data.