"One may formulate this 'third view' of scientific theories briefly by saying that they are genuine conjectures - highly informative guesses about the world which although not verifiable (capable of being shown to be true) can be submitted to severe critical tests. They are serious attempts to discover the truth."
The philosophy of physics was Popper's major concern until the 1960s when he began to write about biological themes. The three volumes of The Postscript to The Logic of Scientific Discovery (mostly written in the 1950s) are overwhelmingly concerned with physics, and this paper was first printed in 1956.
He was mainly concerned with instrumentalism in the philosophy of physics, especially in the Copenhagen interpretation of quantum theory but most of us struggle to understand the issues in this field. However the arguments are equally applicable in other contexts such as the debate in the methods of economics that was triggered by Milton Friedman's espousal of instrumentalism in his famous 1953 paper where he argued that prediction was all-important, regardless of the truth of assumptions used in the calculations.
The chapter has six sections and the first describes the dispute between Galileo and Cardinal Bellarmino before the Inquisition. In the second section he spelled out the issue at stake, whether the new science of celestial mechanics was true (or nearer to the truth than the pre-Copernican system) or whether it was merely a simpler and a more convenient instrument for astronomical calculations and predictions. The point was that the Church did not want science to become a competitor in the field of providing true statements about the cosmos. Bishop Berkeley took the same line in his criticism of Newton's theory because, like Bellarmino before him, he wanted scientific theories to be regarded as convenient instruments for calculation and not as true descriptions of anything real.
The third section of the paper is a critique of essentialism, the first of the three views. The fourth section is a more detailed exposition of instrumentalism and the fifth section is a critique of that view. The sixth section is "The Third View: Conjectures, Truth, and Reality".
"Berkeley's criticism was hardly noticed by the physicists; but it was taken up by the philosophers, sceptical as well as religious. As a weapon it turned out to be a boomerang. In Hume's hands it became a threat to all belief - to all knowledge, whether human or revealed. In the hands of Kant, who firmly believed both in God and in the truth of Newtonian science, it developed into the doctrine that theoretical knowledge of God is impossible, and that Newtonian science must pay for the admission of its claim to truth by the renunciation of its claim to have discovered the real world behind the world of appearance...Physicists (with a few brilliant exceptions such as Mach, Kirchhoff, Hertz, Duhem, Poincare, Bridgeman and Eddington - all instrumentalists) kept aloof from these philosophical debates, which remained completely inconclusive. Faithful to the tradition created by Galileo they devoted themselves to the search for truth, as he had understood it. Or so they did until very recently... Without any further debate over the philosophical issues, without producing any new argument, the instrumantalist view has become an accepted dogma. It may well be called the official view of physical theory since it is accepted by most of our leading theorists of physics (although neither by Einstein nor by Schrodinger). And it has become part of the current teaching in physics."
Returning to the issues at stake in the modern debate: this is the way that instrumentalism has been adopted among physicists by default, without a good discussion, under the influence of developments in quantum physics and especially the Copenhagen interpretation.
Section 3. The First View: Ultimate Explanation by Essences
"(1) The scientist aims at finding a true theory or description of the world which shall also be an explanation of the observable facts...(2) The scientist can succeed in finally establishing the truth of such theories beyond all reasonable doubt...(3)The best, the truly scientific theories, describe the 'essences' or the 'essential natures of things' - the realities which lie behind the appearances."
The first proposition is part of the view that Popper defends. He contests 2 and 3.
Popper famously dissents from (2) in favour of the theory of conjectural knowledge, bearing in mind that the theories here concern deep explanations, not the existence of such things as the evolution of life on earth or the heliocentric structure of the solar system.
As to (3) Popper is not especially concerned to dispute the existence of "essences" that underly the visible appearance of nature. He argues that the essentialist doctrine has tended to stiffle criticism and the search for deeper explanations beyond the current orthodoxy.
Section 4: The Second View: Theories as Instruments
This section includes a comparison of the three views and some exploration of the instrumentalist view, including some considerations of language and meaning that indicate that Popper could have excelled in language analysis if he thought that this was a worthwhile occupation.
Section 5: Criticism of the Instrumentalist View
He summarise the criticism as follows:
"Instrumentalism can be formulated as the thesis that scientific theories - the theories of the so-called 'pure' sciences - are nothing but computation rules (or inference rules); of the same character, fundamentally, as the computation rules of the so-called 'applied' sciences. One might even formulate it as the thesis that 'pure' science is a misnomer, and that all science is applied."
"Now my reply to instrumentalism consists in showing that there are profound differences between pure theories and technological compputation rules, and that instrumentalism can give a perfect description of these rules but is quite unable to account for the difference between them and the theories."
That is the summary of Popper's argument and I don't have time to reproduce more detail from the three pages of compressed argument in this section.
Section 6: The Third View, Conjectures, Truth and Reality
He suggested that instrumentalism is an ad hoc response to some serious problems with the interpretation of quantum theory, and it has the effect of defecting criticism and limiting the field of seach (or even the perceived need) for improved theories that meet the computational requirements and provide a grip on the nature of reality as well.
"There is an important distinction which we can make between two kinds of scientific prediction, and which instrumentalism cannot make; a distinction that is connected with the problem of scientific discovery. I have in mind the distinction between thet prediction of events of a kind which is known, such as eclipses and thunderstorms on the one hand and, on the other, the prediction of new kinds of events (which the physicist calls 'new effects') such as the prediction which led to the discovery of wireless waves, or of zero-point energy, or to the artificial building up of new elements not previously found in nature."
He ends the section with a further criticism of instrumentalism in connection with the alleged denial by instrumentalists of the descriptive function of abstract words and of disposition-words. This leads him to mention some of ideas that emerged in much more detail later, in connection with propensities, both the metaphysics of "a world of propensities", and the propensity theory of probabilities, designed to handle single events rather than frequency distributions, that he developed to contribute to quantum theory.
"I do not think that a language without universals could ever work; and the use of universals commits us to asserting, and thus (at least) to conjecturing, the reality of dispositions - though not of ultimate and inexplicable ones, that is, of essences... But if we are committed, or at least prepared, to conjecture the reality of forces, and of fields of forces, then there is no reason why we should not conjecture that a die has a definite propensity (or a disposition) to fall on one or another of its sides... An interpretation of probability along these lines might allow us to give a new physical interpretation to quantum theory."