This is an illustrated version of a classic of physics-related philosophy, with a carefully prepared clickable table of contents. Sir Arthur Eddington wrote this book about nine years after his popularly-oriented work 'The Nature of the Physical World', and it is a somewhat more technical continuation and elaboration of the same ideas. As he says: "The title of the earlier book might have been expanded into 'the nature of the physical universe, with applications to the theory of physical knowledge'; the corresponding title of the present book would be 'the nature of physical knowledge, with applications to the theory of the physical universe'".
Eddington came to the opinion that (in a manner of speaking) what most of us call "objective" is really subjective, and vice versa. And he claimed to find "no disharmony between a philosophy which embraces the wider significance of human experience and the specialized philosophy of physical science, even though the latter relates to a system of thought of recent growth whose stability is yet to be tested". The book is written in beautiful and mellifluous English. [We have preserved the British spellings for authenticity.] Tidbits: ● If it were necessary to give a short name to this philosophy, I should hesitate between "Selective subjectivism" and "Structuralism". ● You will find plenty of philosophies of objective natural law; you will find here a philosophy of subjective natural law... Primarily the sphere of objective law is the interplay of thoughts, emotions, memories and volitions in consciousness. The purely objective sources of the objective element in our observational knowledge have already been named; they are life, consciousness, spirit. ● "Really exists", for most persons, is a parrot-phrase whose meaning they have not troubled to consider. ● Under cover of the term "good" observation the bed of Procrustes is artfully concealed. ● I would emphasize the question "What is it we really observe?" Because as soon as we ask the question, the classical scheme of physics is a punctured bubble. ● In swallowing up arithmetic, quantum theory has a little overreached itself... it is impossible not to admire the devastating beauty of quantum arithmetic. ● New physical quantities shall be regarded as defined by the series of measuring operations and calculations of which they are the result. Those who associate with the result a mental picture of some entity disporting itself in a metaphysical realm of existence do so at their own risk; physics can accept no responsibility for this embellishment... Quantities such as length and time-extension are introduced solely for the purpose of succinct description of observational measurements actual or hypothetical. ● It is often suggested that some of the constants of nature, e.g. the velocity of light or the gravitational constant, vary with time. Unless the standards of length and time-extension have been carefully defined, such discussions are meaningless; and much that has been written on the subject is discounted by the fact that the writers are evidently unaware of the nature of the definition of these standards... there is no more danger that the velocity of light or the constant of gravitation will change with time than that the circumference-diameter ratio pi will change with time. ● I believe there are 15,747,724,136,275,002,577,605,653,961,181,555,468,044,717,914,527,116,709,366,231,425,076, 185,631,031,296 protons in the universe and the same number of electrons. This number of 80 digits is, as it were, only the grandchild of the number 96. ● In claiming to determine a priori the number of elementary particles in the universe we are not usurping a prerogative which has usually been ascribed to the Creator of the universe... When I speak of the number of particles in the universe, you must not think that I mean that there are N discrete entities, put there by the Creator, ready to be enumerated.