Sir Arthur Eddington, the celebrated astrophysicist, made great strides towards his own 'theory of everything'in his last two books published in 1936 and 1946. Unlike his earlier lucid and authoritative works, these are strangely tentative and obscure - as if he were nervous of the significant advances that he might be making. This volume examines both how Eddington came to write these uncharacteristic books - in the context of the physics and history of the day - and what value they have to modern physics. The result is an illuminating description of the development of theoretical physics, in the first half of the twentieth century, from a unique point of view: how it affected Eddington's thought. This will provide fascinating reading for scholars in the philosophy of science, theoretical physics, applied mathematics and the history of science.