A world of propensities and metaphysical dreams.
During the early 1950s Popper prepared almost a thousand pages of manuscript for publication as a companion volume to the English translation of his Logik Der Forschung (1934). This material started as a series of appendices to The Logic of Scientific Discovery but some of them grew into a book to be called Postscrip to the LSD: After Twenty Years (from 1934 to 1954). The Logic of Scientific Discovery appeared in 1959 but the Postscript lagged behind until eventually Bill Bartley took over the editing and it appeared in three volumes in the 1980s (after 50 years).
In the meantime photocopies of the galleys circulated among Popper's colleagues and this had some impact, especially by way of Imre Lakatos and his "methodology of scientific research programmes" (MSRP), which may well have been inspired by the theory of metaphysical research programs that can be found in The Postscript. Unfortunately, the MSRP, along with unhelpful criticism of Popper by Lakatos caused a great deal of confusion and misplaced effort which might have been avoided if The Postscript and especially Popper's theory of programs had appeared earlier.
The three books of the Postscript are Realism and the Aim of Science" (Volume 1), The Open Universe: An Argument for Indeterminism (Volume 2) and Quantum Theory and the Schism in Physics (Volume 3). They contribute to Popper's long campaign in support of realism, indeterminism and objectivism which in turn support human freedom, creativity and rationality.
Realism has two parts, the first pursues various forms of inductivism and the second attacks the subjective interpretation of the probability calculus. The Open Universe critiques both scientific and metaphysical determinism and traces the linkage between metaphysical determinism and subjective probability theory. This third volume carries the defence of realism and objectivism into the heart of quantum theory to challenge the dominant assumptions of the Copenhagen interpretation. Bartley points out in the editor's introduction that this is a profoundly cosmological work, where "the basic theme of Karl Popper's philosophy - that something can come from nothing - obtains its basis in physics".
The book contains a 'Metaphysical Epilogue' that is remarkable (in addition to being the basis of Lakatos's theory of scientific research programmes) because it provides a key to understanding a set of themes that unify Popper's whole system of thought (the keystone to his arch of thought it you like). This gives some clues as to the depth of his thinking and the reason why it has been so badly received in the profession at large.
Popper's theory of MRPs flows from his theory that we should look at the history of a subject, and its current status, in terms of its problem situations.
"In science, problem situations are the result, as a rule, of three factors. One is the discovery of an inconsistency within the ruling theory. A second is the discovery of an inconsistency between theory and experiment - the experimental falsification of the theory. The third, and perhaps the most important one, is the relation between the theory and what may be called the "metaphysical research programme".
"By raising the problems of explanation which the theory is designed to solve, the metaphysical research programme makes it possible to judge the success of the theory as an explanation. On the other hand, the critical discussion of the theory and its results may lead to a change in the research programme (usually an unconscious change, as the programme is often held unconsciously, and taken for granted), or to its replacement by another programme. These programmes are only occasionally discussed as such: more often, they are implicit in the theories and in the attitudes and judgements of the scientists."
"I call these research programmes "metaphysical" also because they result from general views of the structure of the world and, at the same time, from general views of the problem situation in physical cosmology. I call them "research programmes" because they incorporate, together with a view of what the most pressing problems are, a general idea of what a satisfactory solution of these problems would look like."
The theme of the book is the way that the Copenhagen interpretation of quantum physics has been influenced by unstated and uncriticised metaphysical assumptions, especially determinism, subjectivism and instrumentalism. Of course the Copenhagen people are scientific indeterminists but Popper argues that there is a metaphysical form of determinism that they have not eliminated from their thinking.
The book contains four chapters after a 1982 Preface and an Introduction. The Preface makes a case for a realistic and commonsense interpretation of quantum theory to overcome the crisis in physics which Popper attributes to two things, the intrusion of subjectivism and the "end of the road" idea that quantum theory has reached the complete and final truth. In the Introduction he argues for an interpretation of quantum physics without the observer and he sharply formulated thirteen thesis to challenge the Copenhagen interpretation of the observer as an integral part of the system.
In Chapter I, 'Understanding quantum theory and its interpretations' Popper updated his ideas from the formulations in "The Logic of Scientific Discovery". He still maintained that the problem of interpreting quantum theory is bound up with the interpretation of probability theory, and he argued that the theory of propensities that he described in the first and second volumes of The Postscript should be applied to the interpretation of quantum theory, thus resolving the difficulties that arise in the Copenhagen interpretation.
Chapter II 'The objectivity of qauntum theory' returned to the issue of the observer in the system and confronted the doctrine that experiments have to be interpreted with the observer, and especially the consciousness of the observer, as one of the variables. The discussion includes the nature of quantum jumps and the existence or non-existence of particles.
Chapter III attempts a resolution of the paradoxes of quantum theory, using the propensity interpretation of probability, applied to (1) the indeterminacy relations, (2) the expirement of Einstein, Podolsky and Rosen, and (3) the two-slit experiment.
The long fourth chapter is the Metaphysical Epilogue. This covers a lot of ground, starting with a brief statement of the theory of metaphysical research programs (above). He then ran through a series of ten research programs. First the block universe of Parmenides, then Atomism and Geometrization, followed by Essentialism and Potentialism (from Aristotle), then Renaissance Physics (Copernicus, Bruno, Kepler, Galileo), The Clockwork Theory (Hobbes, Descartes), Dynamism (Newton), Fields of Force (Faraday, Maxwell), Unified Field Theory (Riemann, Einstein, Schrodinger) and finally The Statistical Interpretation of Quantum Theory. After a discussion of schism, programs and metaphysical dreams he went on to indeterminism and the reduction of the wave packet and a model of a universe of propensities to account for the leading featues of all the ten programs that he sketched previously. After touching on some open problems he concluded with some comments on the role of metaphysical systems and the possibility of a demarcation within metaphysics, between good and bad systems.
"The proper aspiration of a metaphysician...is to gather all the true aspects of the world (and not merely its scientific aspects) into a unifying picture which may enlighten him and others, and which may one day become part of a still more comprehensive picture, a better picture, a truer picture."