I've put-off writing this review for over a year.....just because there is so much in the book and, I figured, that it would take me ages to pull my own ideas and responses together. I was right. It is taking me a long time. But that's partly because the book is actually a collection of essays. It was never written as a complete work on its own. And much of the material is repetitive....though most of it fascinating. I've long been an admirer of Karl Popper's work and his thinking. I'm not going to try and write an abstract of the book ....just highlight a few of the gems that I found interesting.
With the sole exception, perhaps, of Protagoras, who seems to argue against it, all serious thinkers before Aristotle made a sharp distinction between knowledge, real knowledge, certain truth (saphes, alétheia; later: epistemö, which is divine and only accessible to the gods, and opinion (doxa), which mortals are able to possess, and is interpreted by Xenophanes as guesswork that could be improved. It seems that the first who revolted against this view was Protagoras; "About the gods we don't know anything, so we don't know what they know. Thus human knowledge must be taken as our standard, as our measure." Yet after Protagoras — but only until Aristotle — most thinkers of importance continued to hold the view of Parmenides and his predecessors that only the gods have knowledge. Popper is critical of Aristotle: "Aristotle killed the critical science to which he himself had made a leading contribution. The philosophy of nature, the theory of nature, the great original attempts in cosmology, broke down after Aristotle, owing mainly to the influence of his epistemology, which demanded proof (including inductive proof).I think this is in brief the story of how epistemology as we know it came to be dominated by what Parmenides would have called a wrong way, the way of induction".
On words and their meaning: "The mistaken (‘essentialist') doctrine that we can define (or explicate) a word or term or concept, that we can make its meaning 'definite' or 'precise', is in every way analogous to the mistaken doctrine that we can prove or establish or justify the truth of a theory; in fact, it is part of the latter (justificationist) doctrine.
Every rational discussion , that is every discussion is based on principles, which in actual fact are ethical principles. I should like to state three of them.
The principle of fallibility. Perhaps I am wrong and perhaps you right, but of course, we may both be wrong.
The principle of rational discussion. We need to test critically and, of course, as impersonally as possible the various (criticizable) theories that are in dispute.
The principle of approximation to truth. We can nearly always come closer to the truth with the help of such critical discussions; and we can nearly always improve our understanding, even in cases where we do not reach agreement.
To quote Boltzmann himself.
For the universe as a whole the two directions of time are indistinguishable, just as in space there is no up and down. But there can be a rare downward movement away from the entropy equilibrium position and time would be experienced here as the the direction of going from the less probably to the more probable. (That is, time could be reversed)....."In spite of the unquestioned victory of the ideas for which Boltzmann fought and died, one cannot say that the situation remains completely satisfactory even now.
Popper has clearly been greatly influenced by part of a poem by Parmenides describing the moon where Parmenides says the round moon points her face to the sun even though the sun is below the horizon and interprets this as Parmenides understanding that the phases of the moon (ie change) actually involved no change in the moon itself....just in the way we observed it......so change was an illusion. He says:
"My hypothesis is that Parmenides' great discovery of the cause of the phases of the Moon shocked and overwhelmed its initiator, who extended it to the entire cosmos. There is nothing unlikely in such a story.
But arguing for his tremendous new message on empirical grounds was not possible for Parmenides. An a priori argument had to be found - a solid proof:
(1) Only what is, is.
(2) The nothing cannot be.
(3) There is no empty space.
(4) The world is full.
(5) Motion and change (which is a kind of motion) are impossible:
(6) There is no room for motion, and thus for change, if the world is full.
This is the goddess's proof; as a proof it is infallible and thus divine. If we look at it as a human achievement, it is staggering. It derives a priori the great empirical discovery of the unmoving Moon, and generalizes it. So his discovery is explained, and with it the cosmos!"
Popper also suggests that Parmenides "way of Truth" might be reconstructed as follows: Premise: Only what is truly the case (such as what is known) can be the case, and can truly be.
First conclusion: The non-existing cannot be.
Second conclusion: Nothingness, or the void, cannot be.
Third conclusion: The world is full: it is a continuous block without any division.
Fourth conclusion: Since the world is full, motion is impossible.
In this way, the cosmology of the goddess, the theory of the block universe, is deductively derived from her theory of genuine knowledge.
though Popper mentions here two of the tenets of Parmenides' theory of knowledge which he regards as mistaken.
I've just pulled a few of the gems from this fascinating work by Popper. Must say that I was blown-away by his casual name dropping about discussions with people like Schrödinger and Einstein.And, I'm also reminded that in a course I completed on Plato and Platonism, my professor warned us against reading Plato's Parmenides ...on the grounds that it was too complex/difficult and required a lot of background knowledge. Having completed the current book, I can understand my professor's wisdom. But the current book would certainly benefit from being re-read. There is a lot there. Happy to give it five stars. ( )

Karl Raimund Popper (Viena, 28 de julio de 1902 - Londres, 17 de septiembre de 1994) fue un filósofo y profesor, nacido en Austria, aunque más tarde se convirtió en ciudadano británico. ( )