The following chapters contain the Gifford Lectures which I was invited to give at the University of Edinburgh from 7th February till 2nd March 1955 The printed text corresponds closely in substance to the lectures as they were delivered. Only minor additions have been made and the number of references to literature increased.
I am conscious that there are many problems which should be discussed further than was possible for me within the framework of these lectures and I must be content if my attempt to deal with them contributes to such further discussion.
I cannot let these lectures be published without saying how deeply grateful I am for the honour of being invited to give the Gifford Lectures and also for the great hospitality and the manifold and helpful kindness which I experienced during the weeks I spent in Edinburgh.
From the book: