People change in many ways. Most changes come about smoothly and gradually, some are abrupt and vehement. In either case the result may be far reaching, as when the change leads to a fundamentally new way of seeing things. Sometimes we are aware of this, albeit perhaps only in retrospect: "Now I look at these things in a completely different way ..." In this book we are particularly interested in such forms of conceptual development by which certain aspects of reality come to be seen from a radically different perspective.