This book must be considered a sequel to the earlier one on special relativity but the difficulties in writing it are of rather a different
order from those described in the introduction to that book. General
relativity, unlike the special theory, is not a general framework but
a specific scientific theory, in fact the best theory of gravitation that
we have to date. It is often said to suffer unduly from lack of experi-
experimental check or confirmation. This fact, however, is essentially due to
the extremely good gravitational theory which we had before, which
was that of Newton's. In the situation when one theory is very good
it is difficult to distinguish between its predictions and those of an
alternative. The main difficulty in writing the book, however, has
been to know what to leave out. Every reader will find the choice
of papers included extremely idiosyncratic, but this is inevitable
when there has been such a sudden rapid advance as has occurred
in general relativity since 1945. If anyone feels that their work has
been passed over, or unfairly treated in any way, I can only offer my
apologies.