We like to think that we plan what happens to us, but it is chance, surely, that lies behind so many of the great events of our lives -- the meeting with the person with whom we are destined to spend the rest of our days, the receiving of a piece of advice whic influences our choice of career, the spotting of a particular house for sale; all of these may be put down to pur chance, and yet they govern how our lives work out and how happy--or unhappy--we were going to be.