  This is a write-up of Sidney Coleman's classic lecture first given as a Dirac
Lecture at Cambridge University and later recorded when repeated at the New
England sectional meeting of the American Physical Society (April 9, 1994). My
sources have been this recording and a copy of the slides Sidney send to me
after he gave the lecture as a Physics Colloquium at Stanford University some
time between 1995 and 1998. To preserve both the scientific content and most of
the charm, I have kept the editing to a minimum, but did add a bibliography
containing the references Sidney mentioned.--Martin Greiter
