Arthur Stanley Eddington (1882–1944), Plumian Professor of Astronomy in Cambridge University for over thirty years, was among the most significant twentieth-century astronomers. He wrote several “popular” books (quite demanding to read) and was a lifelong Quaker. Matthew Stanley, a historian of science, has written about the unity underlying the man's different facets.

By analogy with the valence electrons of atoms, which enable them to combine into molecules, Stanley writes of “valence values” that unite a person's work in different spheres, such as science and religion, usually thought to be distinct or even antagonistic. One of Eddington's principal valence values was “seeking,” as shown by the quotation (one of my own favorites) on page 46 with which Stanley heads a chapter: “You will understand the true spirit neither of science nor of religion unless seeking is placed in the forefront.”

Eddington's emphasis on seeking was part of his Quaker heritage. Stanley describes the late nineteenth-century Quaker Renaissance, at its height during Eddington's adolescence just before he entered Owens College (later the University of Manchester), which similarly emphasized seeking. In Manchester, Eddington was influenced by, among others, one of the leaders of the Renaissance, J. W. Graham, principal of the Quaker hall of residence, Dalton Hall. Echoes of those leaders’ ideas are often present in Eddington's own later work.

Stanley shows how Eddington's spirit of seeking animated his groundbreaking work of the 1920s on stellar structure. Eddington explored how much one could deduce, even without knowledge of the source of stellar energy, in contradistinction to Sir James Jeans, who believed that knowledge to be essential to progress. Readers without a background in astronomy may find this and some other sections heavy going, but they should persevere because it is precisely here that Stanley brings out the essential unity of Eddington's science and religion (the endnotes sometimes help). Stanley himself appears to be equally at home discussing the technical issues involved, Quaker history, or the British attitude to conscientious objectors during the First World War.

The Quaker Eddington was also a pacifist. Early in the war, he was exempted from military service to continue his scientific work, but in 1918 such exemptions were revoked. Both Cambridge University and Sir Frank Dyson, then Astronomer Royal, argued that Eddington's work was sufficiently important to justify renewed exemption, but Eddington himself insisted on his objections of conscience. Stanley describes the treatment of conscientious objectors and stresses that many of Eddington's colleagues disagreed with him, thus clearly bringing out Eddington's moral courage. He displayed that same courage as one of very few who maintained contact (through neutral countries) with German science—one reason why he became the interpreter of relativity to the English-speaking world. Like his pacifism, this internationalism had Quaker roots.

Eddington was finally exempted from call-up, provided that he continued preparations to test, at the total solar eclipse of May 1919, Einstein's prediction that starlight passing near the sun would be deflected—even though no one knew that the war would be over in time. Stanley mentions an earlier attempt to make this test by my own teacher, Freundlich, which was frustrated by the outbreak of the war; at the 1919 eclipse the sun was in a particularly favorable position. Eddington's internationalism and pacifism, Stanley argues, provided his strong motivation to work for reconciliation with the erstwhile enemy. Some have turned that argument around, suggesting that Eddington, consciously or otherwise, selected the observations that gave the most favorable results. Stanley deals with the accusation and, I believe, lays it to rest. I have read the original report and think that I would have made the same selections; but there are valid methodological criticisms, and the very close agreement between theory and observation was partly chance. Stanley notes that physicists are more inclined than astronomers to criticize Eddington's selection, and he suggests that the former are less comfortable with Eddington's later writing on religion. That is an interesting idea, but another factor may be that physicists, used to carefully controlled, repeatable experiments, do not always understand that astronomers must observe unique events, often under less than ideal conditions.

Eddington's involvement in general relativity naturally led him to study cosmology, especially after Hubble's 1929 discovery of the recession of other galaxies. Simultaneously, Eddington became increasingly preoccupied with his “fundamental theory,” an ambitious attempt to show that the fundamental characteristics of the universe could be deduced by pure reason. The theory, always controversial, is now largely ignored, although it has similarities with today's search for a “theory of everything.” Surprisingly, Stanley asserts that neither Eddington's cosmology nor his fundamental theory show much evidence of a connection with his religious beliefs, although this is precisely where we would expect one. I venture to differ, at least as regards fundamental theory. Although, as Stanley points out, Eddington's philosophy of science contains positivist elements, for him mind was prior to matter—a connection, surely, with his belief that the universe includes a spiritual realm. While the rest of the book is clearly and cogently argued, any connection between fundamental theory and Eddington's religious beliefs is dismissed in a brief, almost dogmatic statement.

I also feel that Stanley sometimes dismisses the work of earlier writers too readily, particularly the two biographies The Life of Arthur Stanley Eddington, by A. V. Douglas (London, 1956), and The Eddington Enigma, by D. S. Evans (self-published, 1998). Each book has its limitations, and I may be biased by personal friendships I enjoyed with each author, but I think they deserve a little more credit than they get. Nevertheless, I strongly recommend Stanley's book: he has explored the connection between Eddington's religion and science more deeply than any previous writer, including myself, and his book is essential reading for anyone who wishes to understand the motivations, in both life and science, of its subject. It is also a major contribution to the wider debate about the relationship of science and religion.