ABSTRACT
In this chapter we shall give a rapid review of the remarkable success story of modern physics, which is mainly the physics of the present century. This will help us to see what is meant by an "explanation" in physics; it will show how new facts and new understandings often require quite radical changes in attitude; it should also encourage us to believe that observable phenomena can be understood; finally, although much of this chapter may seem to be irrelevant to our purpose in this book, it is necessary in order to place quantum theory in its proper context.
