“Imagine I were to build a bridge over a stream and for twenty years the bridge stays up and then one day, quite unexpectedly, the bridge collapses. We can ask why the bridge fell down, but the fundamental force of gravity which caused its collapse was always working on the bridge. As we all know from our physics classes, the force of gravity was always active on the bridge and so some other set of forces, channeled through structural elements was needed to be continually resisting that downward pressure. What we really want to know is ‘what force which was keeping the bridge up in such an unnaturally elevated position stopped?’”