John A. Wheeler's two favorite questions are: “How come existence?” and “How come the quantum?” (Wheeler 1998). It is difficult to know how to go about answering the first question. What shape would an answer take? This article is concerned instead with the second question which I will expand as: “Why is nature described by quantum theory?”

What shape would an answer to this question take? We can get a handle on this by considering some historical examples. In the seventeenth century physicists were confronted by Kepler's laws of planetary motion. These laws were empirically adequate for predicting planetary motion and yet sufficiently abstract and ad hoc that they cannot really have been regarded as an explanation of “why” planets move the way they do. Later, Newton was able to show that Kepler's laws followed from a set of reasonable laws for the mechanics (his three laws) plus his law for gravitational forces. At this stage physicists could begin to assert with some degree of confidence that they understood “why” the planets move the way they do. Of course there were still mysteries. Newton was particularly bothered by the action at a distance of his inverse square law. What mediated the force? It was not until Einstein's theory of general relativity that an answer to this question became available.