In chapters 1–11, Eddington explains, in non-technical and sometimes entertaining language, the main ideas of modern physics as it stood in 1927. In chapters 12–15, he then discuses what he considers their main philosophical and religious implications.
The book opens with a short but programmatically important introduction in which Eddington describes two parallel worlds: the world of concrete, sensible reality and the world described by physics. He suggests that modern advances have rendered the latter increasingly detached from the former. Thus, for example, there are simply no ‘counterparts’ or valid ‘analogies’ in the everyday world for the quantum mechanical ‘atoms’ and ‘electrons’ inhabiting the scientific world. These scientific constructs are like the letters in an alphabet. None of these, individually, have ‘counterparts’ in the ‘everyday’ world (consider, for example, the letters ‘c’, ‘a’ and ‘t’); they are symbols out of which we can create words and sentences. The latter may well have ‘counterparts’ in the ‘everyday world’ (e.g., ‘cat’). Eddington argues that physics is concerned with the world of symbols, and not (at least not at present) with building bridges to the ‘everyday’ world.
Thereafter, from chapter I to chapter X, Eddington takes the reader through the theory of special relativity (I, II), thermodynamics, especially the second law (III, IV), general relativity (VI, VII), cosmology (VIII) and quantum mechanics (IX, X). It would be pointless to summarise each of these high-quality (though somewhat dated) expositions in a few short sentences, although it is of some importance to single out what Eddington has to say about the second law of thermodynamics, which states that there exists a measurable property of macroscopic systems, called its ‘entropy’, that always increases in any spontaneous process. Eddington claims that this law ‘holds … the supreme position among the laws of Nature.’ If anyone builds a theory that turns out to contradict the second law of thermodynamics, then ‘there is nothing for it but to collapse in deepest humiliation’ (p. 74). Given the breadth of Eddington’s survey, this is a vote of confidence in the strongest possible terms!
In chapter XI, Eddington summarizes of his thoughts on how to build a grand unified theory (although this terminology is not used). Based as it is on a branch of mathematics found obscure by even many students of mathematical physics (generalised tensor calculus), this is the most difficult of the ‘scientific’ chapters for the general reader.
Eddington’s philosophical interests show through here and there and are more overt in some places then in others. Thus, for example, the very title of chapter V, ‘Becoming’, suggests (correctly) that we may expect a significant dose of meta-scientific analysis here. But on the whole, Eddington deliberately kept explicit philosophy out of the first part of the book, preferring to draw out the philosophical implications ‘in one go’ in the later chapters.
The philosophical chapters are dense with new ideas and novel associations between old ideas. Only a bare outline can be given here. In chapter XII, Eddington argues that physics is about relating numerical quantities that can be measured by instruments. As long as the whole set of quantities referred to in physical theory are defined self-consistently, there is no need to enquire about what they ‘really are’ (e.g., whether the reading on a balance ‘really’ measures ‘quantity of matter’). From this, it follows first that physical knowledge is strictly limited. Of crucial importance to his later argumentation, Eddington stresses that the experimental physicist, in order to measure the quantities the theorist seeks to relate, has to interact with the world with his or her consciousness. Thus, the potentiality of matter to interact with mind is one of matter’s ‘objective properties’. This move enables Eddington in chapter XIII to let the idealist cat fully out of the bag and argue that physical science presupposes a background outside its own scope, and that this background, ‘the substratum of everything’, is mental in character. Eddington calls this background ‘mind-stuff’, though he admits to using both ‘mind’ and ‘stuff’ tendentiously in this nomenclature.
In chapter XIV, Eddington moves on to ‘free will and determinism’, arguing strongly that the (then very recent) advent of Heisenberg’s principle of indeterminacy has broken the stranglehold of strict determinism once and for all. In the fifteenth and final chapter, ‘Science and Mysticism’, Eddington starts by setting side by side a mathematical exposition of wind-generated water waves by Horace Lamb (his teacher at Owens’ College) and a sonnet by Rupert Brooke on the same subject (‘There are waters blown by changing winds to laughter …’), and asks whether, in the light of the former, the sentiments expressed by Brooke in the latter (and shared by Eddington) are mere illusions. To answer his own question, Eddington restates his idealism—the presence of a pervasive mental substratum as background to the self-consistent theorising about measurable quantities. Since we know, from firsthand experience, that our own minds form part of this substratum, there is at least the possibility that within the same substratum there is a greater, ‘universal Mind or Logos’. Eddington therefore argues for the reasonableness (stressing that the latter does not, and cannot ever, constitute proof) of using as the starting point for ‘mystical religion’ certain ‘self-known experiences’. He suggests that these experiences have the character of ‘surrender’. Presumably harking back partly to Brooke’s sonnet, Eddington compares these ‘self-known experiences’ of ‘surrender’ to the way many find themselves ‘[surrendering] to the mystic influence of a scene of natural beauty’.
The volume ends with a short conclusion in which Eddington answers potential criticisms of his philosophical musings.