This book is an attempt to unravel a mystery about the writing of two scientific books by Sir Arthur Eddington, his Relativity Theory of Protons and Electrons of 1936 and his posthumous Fundamental Theory, published ten years later. It is an appropriate time to attempt this, for nearly half a century has elapsed since Eddington's death. There is also a more important reason for this book. The ideas that Eddington thought were behind his books – and it will become clear to what extent these are truly the ideas behind them – were addressed to one specific problem. In the first half of the twentieth century, and certainly from 1926 onwards, physics became rather absurdly divided into two disparate parts. By 1916 general relativity had built a theory of gravitation on top of the successes of the special theory of 1905. It was highly successful on the large scale, notably in astronomy. This whole way of thinking about the world had nothing in common with that of the quantum theory, one form of which started at the turn of the century. Despite its crudity it had considerable success in explaining small scale phenomena. 1926 saw its replacement by a more refined approach but still one totally at variance with that of general relativity. Almost every assumption behind one approach was inconsistent with the other. Eddington was not the only scientist to be concerned about this ridiculous situation. As time has gone on, however, and physics has remained in the same unsatisfactory state, the consensus has been that it is almost indecent to mention the fact.