For example, why does the sun keep shining?  The sun has been around for four billion years, and there was no mechanism which would keep it shining unless neutrinos were involved.  So, whereas it became harder and harder to come to grips with the reality of neutrinos, conceptually, it kept taking an increasing, important role in our understanding of important processes like the sun shining, like radioactivity.  In the late 1950s, the neutrino was becoming an increasingly irritating concept, which we had to come to grips with.  It was confusing us.  There was data that was contradictory; it didn’t make any sense.  There were reactions that should have taken place but didn’t take place.  That’s when a group of us at Columbia came on the idea that we should actually see, try to detect, neutrino collisions.