Thus I see the problem of knowledge in a different way from that of my predecessors. Security and justification of claims to knowledge are not my problem. Instead, my problem is the growth of knowledge: in which sense can we speak of the growth or the progress of knowledge, and how can we achieve it? 
Tag: objective knowledge
What aims we pursue
What is the characteristic difference between a scientific theory and a work of fiction? It is not, I hold, that the theory is possibly true while the descriptions in the story are not true, although truth and falsity have something to do with it. The difference is, I suggest, is that the theory and the story are embedded in different critical traditions. They are meant to be judged by quite different traditional standards (even though these standards might have something in common.)
What characterizes the theory is that it is offered as a solution to a scientific problem; that is, either a problem that has arisen before, in the critical discussion of earlier tentative theories, or (perhaps) a problem that discovered by the author of the theory now being offered, but being discovered within the realm of the problems and solutions belonging to the scientific tradition. 
Against the autonomy of World 3
Popper (1972, 107-108) presents one of his “standard arguments for the independent existence of the third world” with the help of a thought experiment. Compare the following two situations: (a) All our machines and tools are destroyed, and all our knowledge of them. But books and our capacity to learn from them survive; and (b) all our machines and tools are destroyed, and all our knowledge of them. All books are destroyed, too.
In the second case, Popper argues, it would take much more time for the world to reemerge. I think that this hypothesis is quite plausible. Let us assume it is true. What follows from this? According to Popper (1972, 108), his thought experiment demonstrates the “reality, significance, and degree of autonomy of the third world.”
This, however, does not follow from the premises of the thought experiment. Strictly speaking, this thought experiment only demonstrates the significance of books. We may conclude that books, at least some of them, are significant things, which could, under special conditions, decisively influence how the world goes on. If we presuppose, in addition, that books are world 3 objects, then it follows that some world 3 objects are significant things. But the thought experiment itself does not tell us that there is a world 3, and that books are world 3 objects. Books could have great impact yet be physical things. After all, physical things can also significantly change the world, and our life (e.g., the impact of a meteorite).
Popper further illustrates the reality and influence of world 3 by referring to problem situations in science and mathematics. As an example, he presents Brouwer’s invention of his theory of the continuum. He cites the following statement by Heyting about Brouwer’s invention: “If recursive functions had been invented before, Brouwer would perhaps not have formed the notion of a choice sequence” (Popper 1972, 109).
The details of this example do not matter. The example illustrates the point in question. Quite plausibly, problem situations may greatly influence the thinking of scientists. But here, too, we have to distinguish between two questions. The first is whether problem situations exert an influence on world 2, and perhaps on world 1. It is quite another question what problem situations are ontologically. They might be world 3 objects, but they might also belong to worlds 1 or 2. Popper’s example teaches us nothing about this question. It only illustrates the impact of problem situations, not their ontological character. 
The independence of World 3
Among the inmates of my ‘third world’ [World 3] are, more especially, theoretical systems; but inmates just as important are problems and problem situations. And I will argue that the most important inmates of this world are critical arguments, and what may be called—in analogy to a physical state or to a state of consciousness—the state of a discussion or the state of a critical argument; and, of course, the contents of journals, books and libraries. …
Let me repeat one of my standard arguments for the (more or less) independent existence of the third world [World 3].
I consider two thought experiments:
Experiment (1). All our machines and tools are destroyed, and all our subjective learning, including our subjective knowledge of machines and tools, and how to use them. But libraries and our capacity to learn from them survive. Clearly, after much suffering, our world may get going again.
Experiment (2). As before, machines and tools are destroyed, and our subjective learning, including our subjective knowledge of machines and tools, and how to use them. But this time, all libraries are destroyed also, so that our capacity to learn from books becomes useless.
If you think about these two experiments, the reality, significance, and degree of autonomy of the third world [World 3] (as well as its effects on the second and first worlds [Worlds 2 and 1]) may perhaps become a little clearer to you. For in the second case, there will be no re-emergence of our civilization for many millenia. [107-8]
Reducing mental to physical states
If mental entities or, better, mental states should exist—and I myself do not doubt that the do exist—then positing mental states is necessary for any true explanation of them; and shoud they one day be reduced to physical states, then this will be a tremendous success. 
Provoking the ‘belief philosophers’
In upholding an objective third world [World 3] I hope to provoke those whom I call ‘belief philosophers’: those who, like Descartes, Locke, Berkeley, Hume, Kant, or Russell, are interested in our subjective beliefs, and their basis or origin. Against these belief philosophers I urge that our problem is to find better and bolder theories; and that critical preference counts, but not belief. 
The subjectivist blunder of common-sense knowledge
The phenomenon of human knowledge is no doubt the greatest miracle in our universe. It constitutes a problem that will not soon be solved, and I am far from thinking that the preset volume makes even a small contribution to its solution. But I hope that I have helped to restart a discussion which for three centuries has been bogged down in preliminaries.
Since Descartes, Hobbes, Locke, and their school, which includes not only David Hume but also Thomas Reid, the theory of human knowledge has been largely subjectivist: knowledge has been regarded as a specially secure kind of human belief, and scientific knowledge as a specially secure kind of human knowledge.
The essays in this book break with a tradition that can be traced back to Aristotle—the tradition of this commonsense theory of knowledge. I am a great admirer of common sense which, I assert, is essentially selfcritical. But while I am prepared to uphold to the last the essential truth of commonsense realism, I regard the commonsense theory of knowledge as a subjectivist blunder. This blunder has dominated Western philosophy. I have made an attempt to eradicate it, and to replace it by an objective theory of essentially conjectural knowledge. This may be a bold claim but I do not apologize for it. [Preface]
One truth for the Greeks
Herodotus seems to have been one of those somewhat rare people whose minds are broadened by travel. At first he was no doubt shocked by the many strange customs and institutions which he encountered in the Middle East. But he learned to respect them, and to look on some of them critically, and to regard others as the results of historical accidents: he learned to be tolerant, and he even acquired the ability to see the customs and institutions of his own country through the eyes of his barbarian hosts.
This is a healthy attitude. But it may lead to relativism, that is, to the view that there is no absolute or objective truth, but rather one truth for the Greeks, another for the Egyptians, still another for the Syrians, and so on. 
Encouraging an absolutist cast of mind
In their Dialectics of Enlightenment, the German philosophers Horkheimer and Adorno have argued that it was thus no accident that reason so often went hand-in-glove with ‘absolutism’. For reason and science, far from promoting liberty, encourage an absolutist cast of mind, by assuming an ‘absolute’ distinction between true and false, right and wrong, rather than a pluralist diversity of values.