I picked up a copy of Popper Selections after reading Bryan Magee's Confessions of a Philosopher. Magee said that many people knew something about Popper's philosophy but few had actually read any of it. I fell into that category, and Magee's overly enthusiastic promotion of Popper made me want to remedy the situation.

Popper Selections is divided into four major sections: the theory of knowledge, philosophy of science, metaphysics, and social science. The ideas in the first two sections formed a single coherent theory in my understanding, so I haven't separated them in my summary.

For a more literary review of the book, click here. For a summary of my beliefs, click here (or read to the end of this page).

• Theory of Knowledge
• Metaphysics
• Social Science

"But as for certain truth, no man has known it,
Nor will he know it; neither the gods,
Nor yet of all the things of which I speak.
And even if by chance he were to utter
The final truth, he would himself not know it:
For all is but a woven web of guesses."
Xenophanes

Popper's epistemology addresses itself primarily to the problems associated with the tradition of empiricism. Empiricism claims that all knowledge is founded on personal observation of sensory experience; we build our understanding of the world by observing regularities and extrapolating ideas from these sensory experiences. Furthermore, empiricism says that sensory experiences must be the basis for knowledge, because they are the only experiences we have of the world.

The fundamental question that plagues empiricism is how we can establish the truth of universal laws based on a limited number of experiences. Newton proposed his theory of gravity based on observation of the planets in our solar system. How can we know (as opposed to believe) that gravity applies throughout the universe for all time? After all, the orbit of the planets could change tomorrow, or gravity could be a feature just of our local neighborhood.

The process by which we extrapolate general laws from specific instances is called induction. The fundamental question of empiricism is "How is induction possible?" Without induction, we can never establish the truth of our knowledge—and established truth is what separates knowledge from belief.

Popper believes that we can never know the absolute truth. We can only make approximations to it. The only way to make progress, to get closer to the truth, is to come up with imaginative theories of how the world works, then put these theories to the toughest possible tests. If our theories pass these tests, then they are likely to be good approximations of the truth. This principle (among others) also gives us a way of comparing theories, which is what makes progress possible.

"Hume's negative result establishes for good that all our universal laws or theories remain for ever guesses, conjectures, hypotheses. But the second negative result concerning the force of counterinstances by no means rules out the possibility of a positive theory of how, by purely rational arguments, we can prefer some competing conjectures to others."
The Problem of Induction, page 111

Popper calls his approach critical rationality. He traces the birth of philosophy, and indeed of Western civilization, to the Greek schools that taught its students to criticize and attempt to improve on the teachings of the master. Previous schools, and most schools of thought today, work to pass on an accepted body of doctrine and are intolerant of criticism.

He is a strong believer in rationality; however, he believes that unconstrained rationality leads to contradiction.

"The rationalist attitude is characterized by the importance it attaches to argument and experience. But neither logical argument nor experience can establish the rationalist attitude; for only those who are ready to consider argument or experience, and who have therefore adopted this attitude already, will be impressed by them. … Thus a comprehensive rationalism is untenable.

But this means that whoever adopts the rationalist attitude does so because he has adopted, consciously or unconsciously, some proposal, or decision, or belief, or behaviour; an adoption which may be called 'irrational.'… We may describe it as an irrational faith in reason."
The Defence of Rationalism, page 35

The goal of scientific practice, in Popper's view, is to make every effort to falsify the theory. Prima facie, this idea sounds like a minor variation on the central tenet of logical positivism—that the goal of scientific practice is to verify theories by providing an experiential basis for them. Many people misunderstand Popper in this way, and believe that he is essentially a logical positivist who simply replaced the positivist principle of verification with his principle of falsification. On the contrary, Popper is quite hostile toward the positivists, and more generally toward empiricism. Most critically, he believes that our knowledge is never ever based on experience or observation.

On a practical level, he points out that when we go to verify a statement, we don't (except in the case of history) try to trace it back to the original experience or observation. "The investigator will, if possible, try to check, or to examine, the asserted fact itself, rather than trace the source of the information" (p. 47-48). In other words, we try to find a means of testing the assertion. This fact resolves a well-known problem of empiricism, namely the fallibility of even our senses. We don't accept facts because of a solid source, but because it stands up to tests.

"So my answer to the questions 'How do you know? What is the source or the basis of your assertion? What observations have led you to it?' would be: 'I do not know: my assertion was merely a guess. Never mind the source, or the sources, from which is may spring-there are many possible sources, and I may not be aware of half of them; and origins or pedigrees have in any case little bearing upon truth. But if you are interested in the problem which I tried to solve by my tentative assertion, you may help me by criticizing it as severely as you can; and if you can design some experimental test which you think can refute my assertion, I shall gladly, and to the best of my powers, help you to refute it."
Knowledge Without Authority, page 53

On a philosophical level, Popper doesn't believe that induction exists. In fact, he goes so far as to say that empirical evidence is not and can never be the basis for a theory or law.

"The fate of a theory, its acceptance or rejection, is decided by observation and experiment-by the results of tests. So long as a theory stands up to the severest tests we can design, it is accepted; if it does not, it is rejected. But it is never inferred, in any sense, from the empirical evidence. There is neither a psychological nor a logical induction. Only the falsity of a theory can be inferred from empirical evidence, and this inference is a purely deductive one."
The Problem of Induction, page 102

"Every test of a theory, whether resulting in its corroboration or falsification, must stop at some basic statement or other which we decide to accept. … Any basic statement can again in its turn be subjected to tests … This procedure has no natural end. …
It is fairly easy to see that we arrive in this way at a procedure according to which we stop only at a kind of statement that is especially easy to test. For it means we are stopping at statements about whose acceptance or rejection the various investigators are likely to reach agreement. …
Statements about personal experiences—i.e. protocol sentences—are clearly not of this kind; thus they will not be very suitable to serve as statements at which we stop. … In general, and especially '…in critical cases' we so stop at easily testable statements and not, as Carnap recommends, at perception or protocol sentences."
The Empirical Basis, page 160

Although he concludes that our knowledge is never certain, Popper sees his worldview as an optimistic one. It reserves a central role for the human imagination, and it allows for an idea of progress toward the truth.

"We must not forget that the function of the Baconian myth is to explain why scientific statements are true, by pointing out that observation is the 'true source' of our scientific knowledge. Once we realize that all scientific statements are hypotheses, or guesses, or conjectures, and that the vast majority of these conjectures (including Bacon's own) have turned out to be false, the Baconian myth becomes irrelevant. For it is pointless to argue that the conjectures of science…all start from observation.
There can be no doubt whatever that Anaximander's theories are critical and speculative rather than empirical: and considered as approaches to truth his critical and abstract speculations served him better than observational experience or analogy.
But, a follower of Bacon may reply, this is precisely why Anaximander was not a scientist… Yet there is the most perfect possible continuity of thought between the theories of the Presocratics and the later developments in physics."
Cosmology and Change, page 227-231

"Every time we proceed to explain some conjectural law or theory by a new conjectural theory of a higher degree of universality, we are discovering more about the world, trying to penetrate deeper into its secrets. And every time we succeed in falsifying a theory of this kind, we make an important new discovery. For these falsifications are most important. They teach us the unexpected; and they reassure us that, although our theories are made by ourselves, although they are our own inventions, they are none the less genuine assertions about the world; for they can clash with something we never made."
The Aim of Science, page 167

For example, "it was one of the greatest achievements of our time when Einstein showed that, in light of experience, we may question and revise our presuppositions regarding even space and time" (page 375).

"Call us negativists or what you like: but you should realize that we are as much interested in truth as anybody-for example, as the members of a court of justice. When the judge tells a witness that he should speak 'the truth, the whole truth, and nothing but the truth,' then what he looks for is as much of the relevant truth as the witness may be able to offer…. Thus when we stress that we are not interested in mere truth but in interesting and relevant truth, then, I contend, we only emphasize a point which everyone accepts. And if we are interested in bold conjectures, even if these should soon turn out to be false, then this interest is due to our methodological conviction that only with the help of such bold conjectures can we hope to discover interesting and relevant truth."
Truth and Approximation to Truth, page 191

An interesting historical note: Popper sees his philosophy of critical rationalism as an extension of Kant's. He believes that Kant didn't take the step because of the "almost unbelievable success" of Newton's theory in passing tests. Until Einstein everyone believed Newton's theory was the Truth.

Popper defines as metaphysical (or philosophical) any theory that is irrefutable. Since it is irrefutable-and no theory at all can be verified-no metaphysical theory can be proven or disproven. However, as with scientific theories, he believes there are rational reasons for preferring one theory to another. By applying these reasons, he concludes that we must prefer realism (the belief that the real world exists independently of our experience) to idealism (the belief that the world is our construction). The main reason is the intersubjective testability of our reality, which he elaborates on with a quote from Winston Churchill:

"Here is this great sun standing apparently on no better foundation than our physical senses. But happily there is a method, apart altogether from our physical senses, of testing the reality of the sun…astronomers…predict by [mathematics and] pure reason that a black spot will pass across the sun on a certain day. … We have taken what is called in military map-making 'a cross-bearing.' We have got independent testimony to the reality of the sun. When my metaphysical friends tell me that the data on which the astronomers made their calculations…were necessarily obtained originally through the evidence of their senses, I say 'No.' They might, in theory at any rate, be obtained by automatic calculating machines set in motion by the light falling upon them without admixture of the human senses at any stage."
Winston Churchill

As Popper says, though, "Churchill's argument is merely an excellent refutation of the specious arguments of the subjectivists: he does not prove realism" (page 225).

Popper is an indeterminist, meaning he doesn't believe that the physical laws of the universe predetermine everything in clockwork fashion. In fact, he believes that even the best understood phenomena have an element of chance in them.

"I shall assume that we have before us a schema or arrangement in which a very disturbed or disorderly cloud is placed on the left. At the other extreme of our arrangement, on its right, we may place a very reliable pendulum clock, a precision clock, intended to represent physical systems which are regular, orderly, and highly predictable in their behavior…
One of the things which almost everyone thought had been established by the Newtonian revolution was the following staggering proposition:
All clouds are clocks—even the most cloudy of clouds. …
The physical determinist who says that all clocks are clouds will also say that our commonsense arrangement, with the clouds on the left and the clocks on the right, is misleading, since everything ought to be placed on the right. He will say that, with all our common sense, we arranged things not according to their nature, but merely according to our ignorance. …
Newton's theory did not, of course, tell the physicists that this was so. In fact, it did not treat at all of clouds… Newton's theory was the first really successful scientific theory in human history; and it was tremendously successful. … All openminded men…were converted to the new theory. …[and] thought that in the end it would explain everything."
Indeterminism and Human Freedom, page 247, 250

He goes on to conclude that not only are clouds not clocks, but all clocks are clouds. He identifies the source of the indeterminacy as occurring at the quantum level. In this connection, he makes an interesting point about the complexity of understanding any situation in terms of laws of nature:

"Although we may assume that any actual succession of phenomena proceeds according to the laws of nature, it is important to realize that practically no sequence of, say, three or more casually connected concrete events proceeds according to any single law of nature. If the wind shakes a tree and Newton's apple falls to the ground, nobody will deny that these events can be described in terms of causal laws. But there is no single law, such as that of gravity, nor even a single definite set of laws, to describe the actual succession of events."
Historicism, page 302

Popper also points out that simply saying that there is indeterminacy in the universe isn't enough to explain our ability to have an impact on the world through our decisions.

"I am a physical indeterminist. …We have to be indeterminists; yet I shall try to show that indeterminism is not enough. …
If determinism is true, then the whole world is a perfectly running flawless clock, including all clouds, all organisms, all animals, and all men. If, on the other hand, Pierce's or Heisenberg's or some other form of indeterminism is true, then sheer chance plays a major role in our physical world. But is chance really more satisfactory than determinism?"
Indeterminism and Human Freedom, page 261

In Popper's view, we need to explain how our purposes and decisions can exert "downward causality" on the physical world.

"We may speak of downward causation whenever a higher structure operates causally upon its substructure. The difficulty of understanding downward causation is this. We think we can understand how the substructures of a system cooperate to affect the whole system; that is to say, we think that we understand upward causation. But the opposite is very difficult to envisage. … It is this that leads to the heuristic demand that we explain everything in terms of molecular or other elementary particles (a demand that is sometimes called 'reductionism').
I suggest that downward causation can sometimes at least be explained as selection operating on the randomly fluctuating elementary particles. The randomness of the movements of the elementary particles … provides, as it were, the opening for the higher-level structure to interfere. A random movement is accepted when it fits into the higher level structure; otherwise it is rejected."
Natural Selection and Its Scientific Status, page 245

Popper proposes a plane of existence that he calls "World 3," which contains the "objective contents of thoughts." That is, he grants objective existence to ideas and theories. Why? Because they can influence events in world 2 (the world of our subjective experience) and world 1 (the physical world).

"Let me repeat one of my standard arguments for the (more or less) independent existence of 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."
Knowledge: Subjective Versus Objective, page 59

With thought experiments like these, Popper defends the existence of objective knowledge, that it, knowledge without a knower. Another argument involves the discovery of a previously unnoticed contradiction in a theory of Frege's by Bertrand Russell. The theory existed for many years before anyone noticed it, but it was there all the time.

"World 3 objects and their properties and relations cannot be reduced to world 2 objects. Nor can they be reduced to brain states or dispositions; not even if we were to admit that all mental states and processes can be reduced to brain states and processes. This is so despite the fact that we can regard world 3 as the product of human minds.
Russell did not invent or produce the inconsistency, but he discovered it. … Had Frege's theory not been objectively inconsistent, he could not have applied Russell's inconsistency proof to it… Thus the state of Frege's mind (and no doubt also a state of Frege's brain) were the result, partly, of the objective fact that this theory was inconsistent."
The Mind-Body Problem, page 269

Popper makes an interesting analogy between our production of ideas and theories and the non-living structures that other animals produce.

"A biologist may be interested in the behavior of animals; but he may also be interested in some of the non-living structures which animals produce, such as spiders' webs, or nests built by wasps or ants, the burrows of badgers, dams constructed by beavers, or paths made by animals in forests. I will distinguish between two main categories of problems arising from the study of these structures. The first category consists of problems concerned with the methods used by the animals, or the ways the animals behave when constructing these structures. … The second category of problems is concerned with the structures themselves. It is concerned with the chemistry of the materials used in the structure; with their geometrical and physical properties; with their evolutionary changes, depending on special environmental conditions … Very important also is the feedback relation from the properties of the structure to the behavior of the animals."
Knowledge: Subjective Versus Objective, page 64

These structures undoubtedly exist; they exist independently of the animals that produce them, yet they would not exist if the animals didn't. In other words, these structures show world 2 and world 3 interacting.

Popper's views in social philosophy bear an important resemblance to his views in the philosophy of science. He believes that the only way to improve human society is through a process of trial and error, critically examining our policies in the same way scientists critically examine theories. Therefore, he believes that only an "open society" that encourages debate and freedom of expression can improve the lives of its citizens. He is a harsh critic of Utopian social programs, especially Marxism.

He points out that the social sciences have always striven for the legitimacy of the physical sciences. He believes, though, that they have misunderstood how the physical sciences work.

"It has not always been realized that two different kinds of predictions can be distinguished in science, and accordingly two different ways of being practical. We may predict (1) the coming of a typhoon, a prediction which may be of the greatest practical value because it may enable people to take shelter in time; but we may also predict (2) that if a certain shelter is to stand up to a typhoon, it must be constructed in a certain way …

The two kinds of predictions are obviously very different… In one case we are told about an event which we can do nothing to prevent. I shall call such a prediction a 'prophecy.' … Opposed to these are predictions of the second kind which we can describe as technological predictions since predictions of this kind form a basis of engineering."
Historicism, page 293

Popper believes that Utopian social programs such as Marxism tend toward prophecy, whereas he advocates "a methodology which aims at technological social science" (page 295). He calls his approach "piecemeal social engineering." He does not believe that we can learn from holistic social experiments:

"It is very hard to learn from very big mistakes…Since so much is done at a time, it is impossible to say which particular measure is responsible for any of the results; or rather, if we do attribute a certain result to a certain measure, then we do so on the basis of some theoretical knowledge gained previously."
Piecemeal Social Engineering, page 315

We end up doing piecemeal engineering anyway, but in more difficult circumstances. He believes that the proper role of politics is to ask the question: "How can we so organize political institutions that bad or incompetent rulers be prevented from doing much damage?" (page 320).

Popper has other objections to Utopian schemes. For example, they typically look for a "law of evolution" that can explain (and predict) the development of society. No such law exists, even in the physical world (see the quote above about sequences of events). Also, Utopian schemes inevitably depend on changing human nature.

He argues that sociology is an independent field from psychology using much the same kind of argument he advances for the existence of world 3:

"Our actions are to a very large extent explicable in terms of the situation in which they occur. … The 'psychological' part of the explanation is very often trivial, as compared with the detailed determination of his action by what we may call the logic of the situation. … In fact, the psychological analysis of an action in terms of its (rational or irrational) motives presupposes-as has been pointed out by Max Weber-that we have previously developed some standard of what is to be considered as rational in the situation in question."
The Autonomy of Sociology, page 353-354

While Popper admits that people's social background can influence their outlook, he sees critical rationality as a way out of believing that truth is socially conditioned.

"If scientific objectivity were founded, as the sociologistic theory of knowledge naively assumes, upon the individual scientist's impartiality or objectivity, then we would have to say goodbye to it. … Ironically enough, objectivity is closely bound up with the social aspect of scientific method, with the fact that science and scientific objectivity do not (and cannot) result from the attempts of an individual scientist to be 'objective,' but from the friendly-hostile co-operation of many scientists." Against the Sociology of Knowledge, page 371-372

I essentially agree with all of Popper's ideas in the theory of knowledge. We can't ever know anything for sure, but successive theories do bring us closer to the truth. When evaluating a claim, we should test it any way we can and not concern ourselves with the source of the claim (in experience or imagination). The source of scientific theories is our imagination not empirical facts, because empirical facts are consistent with an infinite number of theories. (I like the importance of imagination in the theory.) I am sure others can—and have—improved on Popper's account of how science progresses, but I am convinced by his fundamental ideas. Philosophers I've read who have enlarged the view of science include Thomas Kuhn and W.V.O. Quine; a philosopher I should read on this subject is Paul Feyerabend.

Popper's writings on metaphysics don't seem particularly deep to me. I agree that metaphysical theories (by definition) can't be proven or refuted, and I agree that we can devise criteria for choosing the preferable ones. However, I don't know that we can all agree on the criteria. On the subject of determinism, my natural inclination is to be an indeterminist, but logically I can't reconcile it with my belief that the universe operates by general laws. I don't feel like Popper helped me to resolve this personal dilemma.

I was intrigued, though, by Popper's arguments for the objective existence of world 3, consisting of ideas, theories, and such. Before reading this book, I would have dismissed this suspiciously Platonic idea because it postulates the existence of invisible abstract objects whose relationship to the "real" world raises problems comparable to the mind-body problem. But I found Popper's arguments convincing. I need to think about it more. The idea gives me a new perspective on the structuralist movement in linguistics, which focused on language as a system, independent of its speakers. I've always felt that the psychological approach, focusing on language in the heads of its speakers, was the only real way to go. Popper's analogy about beavers and dams makes me rethink my beliefs.

Popper made another observation that I found very interesting from the point of view of my ideas in linguistics:

"While we may say that the essentialist interpretation reads a definition 'normally,' that is to say, from the left to the right, we can say that a definition, as it is normally used in modern science, must be read back to front, or from the right to the left; for it starts with the defining formula, then asks for a short label for it. Thus the scientific view of the definition 'A puppy is a young dog' would be that it is an answer to the question 'What shall we call a young dog?' rather than an answer to the question 'What is a puppy?' … The scientific use of definitions, characterized by the approach 'from the right to the left,' may be called its nominalist interpretation, as opposed to its Aristotelian or essentialist interpretation. … And we can at once see from this that definitions do not play any very important part in science. For shorthand symbols can always be replaced by the longer expressions for which they stand."
Two Kinds of Definitions, page 92

I never thought about this particular difference between nominalists (like myself) and essentialists.

I agree with a lot of Popper's criticisms of existing social schemes, but I'm not sure I'm convinced that his "piecemeal engineering" works for me. It sounds too conservative; I think I believe that revolutionary measures are appropriate in some cases. I have to admit, though, that I haven't thought a lot about social or political philosophy.