In the opening days of September, 1939, just as the Second
World War was breaking out, an International Congress for the
Unity of Science was held at Cambridge, Massachusetts. The
committee in charge of that congress invited me to submit a
paper there. This invitation suggested that I write out, in a more
or less systematic form, the philosophical opinions which I had
formed in a little more than a decade of study in mathematical
logic. I found the task a longer and more difficult one than I had
anticipated; nevertheless I persisted in it. The resulting manuscript
was, of course, too long for presentation at the congress; but it
formed the basis of the paper presented there, and of several other
papers, cited below, which have been written since