Editing Karl Popper (section)

=== Metaphysics ===
==== Truth ====
As early as 1934, Popper wrote of the search for truth as "one of the strongest motives for scientific discovery."<ref>{{Cite news |last=Williams |first=Liz |date=10 September 2012 |title=Karl Popper, the enemy of certainty, part 1: a rejection of empiricism |work=The Guardian |url=https://www.theguardian.com/commentisfree/2012/sep/10/karl-popper-enemy-uncertainty |access-date=22 February 2014}}</ref> Still, he describes in ''Objective Knowledge'' (1972) early concerns about the much-criticised notion of [[Correspondence theory of truth|truth as correspondence]]. Then came the [[semantic theory of truth]] formulated by the logician [[Alfred Tarski]] and published in 1933. Popper wrote of learning in 1935 of the consequences of Tarski's theory, to his intense joy. The theory met critical objections to [[truth]] as correspondence and thereby rehabilitated it. The theory also seemed, in Popper's eyes, to support [[metaphysical realism]] and the regulative idea of a search for truth.

According to this theory, the conditions for the truth of a sentence as well as the sentences themselves are part of a [[metalanguage]]. So, for example, the sentence "Snow is white" is true if and only if snow is white. Although many philosophers have interpreted, and continue to interpret, Tarski's theory as a [[Deflationary theory of truth|deflationary theory]], Popper refers to it as a theory in which "is true" is replaced with "[[correspondence theory|corresponds to the facts]]". He bases this interpretation on the fact that examples such as the one described above refer to two things: assertions and the facts to which they refer. He identifies Tarski's formulation of the truth conditions of sentences as the introduction of a "metalinguistic predicate" and distinguishes the following cases:

# "John called" is true.
# "It is true that John called."

The first case belongs to the metalanguage whereas the second is more likely to belong to the object language. Hence, "it is true that" possesses the logical status of a redundancy. "Is true", on the other hand, is a predicate necessary for making general observations such as "John was telling the truth about Phillip."

Upon this basis, along with that of the logical content of assertions (where logical content is inversely proportional to probability), Popper went on to develop his important notion of [[verisimilitude]] or "truthlikeness". The intuitive idea behind verisimilitude is that the assertions or hypotheses of scientific theories can be objectively measured with respect to the amount of truth and falsity that they imply. And, in this way, one theory can be evaluated as more or less true than another on a quantitative basis which, Popper emphasises forcefully, has nothing to do with "subjective probabilities" or other merely "epistemic" considerations.

The simplest mathematical formulation that Popper gives of this concept can be found in the tenth chapter of ''Conjectures and Refutations''. Here he defines it as:

: <math>\mathit{Vs}(a)=\mathit{CT}_v(a)-\mathit{CT}_f(a) \,</math>

where <math>\mathit{Vs}(a)</math> is the verisimilitude of ''a'', <math>\mathit{CT}_v(a)</math> is a measure of the content of the truth of ''a'', and <math>\mathit{CT}_f(a)</math> is a measure of the content of the falsity of ''a''.

Popper's original attempt to define not just verisimilitude, but an actual measure of it, turned out to be inadequate. However, it inspired a wealth of new attempts.{{sfn|Thornton|2015}}

==== Popper's three worlds ====
{{Main|Popper's three worlds}}
Knowledge, for Popper, was objective, both in the sense that it is objectively true (or truthlike), and also in the sense that knowledge has an ontological status (i.e., knowledge as object) independent of the knowing subject (''Objective Knowledge: An Evolutionary Approach'', 1972). He proposed [[Popper's three worlds|three worlds]]:<ref>Popper, Karl, "Three Worlds, The Tanner Lecture on Human Values", The University of Michigan, 1978.</ref> '''World One''', being the physical world, or physical states; '''World Two''', being the world of mind, or individuals' private mental states, ideas and perceptions; and '''World Three''', being the ''public'' body of human knowledge expressed in its manifold forms (e.g., "scientific theories, ethical principles, characters in novels, philosophy, art, poetry, in short our entire cultural heritage"<ref>[[Mario Vargas Llosa|Vargas Llosa, Mario]], ''The Call of the Tribe'' (''La llamada de la tribu'', 2018), trans. John King (New York: Farrar, Straus and Giroux, 2023), "Sir Karl Popper (1902–1994)", p. 148.</ref>), or the products of World Two made manifest in the materials of World One (e.g., books, papers, paintings, symphonies, cathedrals, [[particle accelerator]]s). World Three, Popper argued, was the product of individual human beings in exactly the same sense that an animal path in the jungle is the creation of many individual animals but not planned or intended by any of them. World Three thus has an existence and an evolution independent of any individually known subjects. The influence of World Three on the individual human mind (World Two) is in Popper's view at least as strong as the influence of World One. In other words, the knowledge held by a given individual mind owes at least as much to the total, accumulated wealth of human knowledge made manifest as to the world of direct experience. As such, the growth of human knowledge could be said to be a function of the independent evolution of World Three.

Many contemporary philosophers, such as [[Daniel Dennett]],<ref>Dennett, Daniel C., [https://dl.tufts.edu/pdfviewer/3x817009m/5712mk336 review of Karl R. Popper and John C. Eccles, ''The Self and Its Brain''], in ''[[The Journal of Philosophy]]''. '''76''' (2): 91–97. Retrieved 12 May 2025.</ref> have not embraced Popper's Three World conjecture, mostly due to what they see as its resemblance to [[mind–body dualism]].<ref>{{cite web |title=Karl Popper (Stanford encyclopedia) |url=https://plato.stanford.edu/entries/popper/ |website=Stanford encyclopedia |access-date=14 February 2025}}</ref>

==== Origin and evolution of life ====
The [[creation–evolution controversy]] raised the issue of whether creationistic ideas may be legitimately called science. In the debate, both sides and even courts in their decisions have invoked Popper's criterion of falsifiability (see [[Daubert standard]]). In this context, passages written by Popper are frequently quoted in which he speaks about such issues himself. For example, he famously stated "[[Darwinism]] is not a testable scientific theory, but a [[Metaphysics|metaphysical]] research program—a possible framework for testable scientific theories." He continued:
{{blockquote|And yet, the theory is invaluable. I do not see how, without it, our knowledge could have grown as it has done since Darwin. In trying to explain experiments with [[bacteria]] which become adapted to, say, [[penicillin]], it is quite clear that we are greatly helped by the theory of [[natural selection]]. Although it is metaphysical, it sheds much light upon very concrete and very practical researches. It allows us to study adaptation to a new environment (such as a penicillin-infested environment) in a rational way: it suggests the existence of a mechanism of adaptation, and it allows us even to study in detail the mechanism at work.<ref>''Unended Quest'' ch. 37 – see Bibliography</ref>}}

He noted that [[theism]], presented as explaining adaptation, "was worse than an open admission of failure, for it created the impression that an ultimate explanation had been reached".<ref name="popper_on_natural_selection">{{Cite web |date=2 November 2005 |title=CA211.1: Popper on natural selection's testability |url=http://www.talkorigins.org/indexcc/CA/CA211_1.html |access-date=26 May 2009 |publisher=[[talk.origins]]}}</ref> Popper later said:
{{blockquote|When speaking here of Darwinism...This is an immensely impressive and powerful theory. The claim that it completely explains evolution is of course a bold claim, and very far from being established. All scientific theories are conjectures, even those that have successfully passed many severe and varied tests. The [[Mendelian inheritance|Mendelian underpinning]] of modern Darwinism has been well tested, and so has the theory of evolution....<ref name="popper_on_natural_selection" />}}

He explained that the difficulty of testing had led some people to describe natural selection as a [[tautology (logic)|tautology]], and that he too had in the past described the theory as "almost tautological", and had tried to explain how the theory could be untestable (as is a tautology) and yet of great scientific interest:
{{blockquote|My solution was that the doctrine of natural selection is a most successful metaphysical research programme. It raises detailed problems in many fields, and it tells us what we would expect of an acceptable solution of these problems. I still believe that natural selection works in this way as a research programme. Nevertheless, I have changed my mind about the testability and logical status of the theory of natural selection; and I am glad to have an opportunity to make a recantation.<ref name="popper_on_natural_selection" />}}

Popper summarised his new view as follows:
{{blockquote|The theory of natural selection may be so formulated that it is far from tautological. In this case it is not only testable, but it turns out to be not strictly universally true. There seem to be exceptions, as with so many biological theories; and considering the random character of the variations on which natural selection operates, the occurrence of exceptions is not surprising. Thus not all phenomena of evolution are explained by natural selection alone. Yet in every particular case it is a challenging research program to show how far natural selection can possibly be held responsible for the evolution of a particular organ or behavioural program.<ref>{{Cite book |last1=Radnitzky |first1=Gerard |url=https://books.google.com/books?id=QnFiTrCzg5oC&pg=PA145 |title=Evolutionary Epistemology, Rationality, and the Sociology of Knowledge |last2=Popper |first2=Karl Raimund |publisher=Open Court |year=1987 |isbn=978-0-8126-9039-2 |access-date=12 August 2014}}</ref>}}

These frequently quoted passages are only a small part of what Popper wrote on evolution, however, and may give the wrong impression that he mainly discussed questions of its falsifiability. Popper never invented this criterion to give justifiable use of words like science. In fact, Popper stressed that "the last thing I wish to do, however, is to advocate another dogma"<ref>LScD, preface to the first english edition</ref> and that "what is to be called a 'science' and who is to be called a 'scientist' must always remain a matter of convention or decision."<ref>LScD, section 10</ref> He quotes Menger's dictum that "Definitions are dogmas; only the conclusions drawn from them can afford us any new insight"<ref>LScD, section 11</ref> and notes that different definitions of science can be rationally debated and compared:
{{blockquote|I do not try to justify [the aims of science which I have in mind], however, by representing them as the true or the essential aims of science. This would only distort the issue, and it would mean a relapse into positivist dogmatism. There is only ''one'' way, as far as I can see, of arguing rationally in support of my proposals. This is to analyse their logical consequences: to point out their fertility—their power to elucidate the problems of the theory of knowledge.<ref>LScD, section 4</ref>}}

Popper had his own sophisticated views on evolution<ref>Niemann, Hans-Joachim: Karl Popper and the Two New Secrets of Life: Including Karl Popper's Medawar Lecture 1986 and Three Related Texts.  Tubingen: Mohr Siebeck, 2014. {{ISBN|978-3161532078}}.</ref> that go much beyond what the frequently-quoted passages say.<ref>For a secondary source see H. Keuth: ''The philosophy of Karl Popper'', section 15.3 "World 3 and emergent evolution". See also John Watkins: Popper and Darwinism. ''The Power of Argumentation'' (Ed Enrique Suárez Iñiguez). Primary sources are, in particular,
* ''Objective Knowledge: An evolutionary approach'', section "Evolution and the Tree of Knowledge";
* ''Evolutionary epistemology'' (Eds. G. Radnitzsky, W.W. Bartley), section "Natural selection and the emergence of mind";
* ''In search of a better world'', section "Knowledge and the shaping of rationality: the search for a better world", p. 16;
* ''Knowledge and the Body-Mind Problem: In Defence of Interaction'', section "World 3 and emergent evolution";
* ''A world of propensities'', section "Towards an evolutionary theory of knowledge"; and
* ''The Self and Its Brain: An Argument for Interactionism'' (with John C. Eccles), sections "The biological approach to human knowledge and intelligence" and "The biological function of conscious and intelligent activity".</ref> In effect, Popper agreed with some points of both creationists and naturalists, but disagreed with both on crucial aspects. Popper understood the universe as a creative entity that invents new things, including life, but without the necessity of something like a god, especially not one who is pulling strings from behind the curtain. He said that evolution of the genotype must, as the creationists say, work in a goal-directed way<ref>D. W. Miller: Karl Popper, a scientific memoir. ''Out of Error'', p. 33</ref> but disagreed with their view that it must necessarily be the hand of god that imposes these goals onto the stage of life.

Instead, he formulated the spearhead model of evolution, a version of genetic pluralism. According to this, living organisms have goals, and act according to these goals, each guided by a central control. In its most sophisticated form, this is the brain of humans, but controls also exist in much less sophisticated ways for species of lower complexity, such as the [[amoeba]]. This control organ plays a special role in evolution—it is the "spearhead of evolution". The goals bring the purpose into the world. Mutations in the genes that determine the structure of the control may then cause drastic changes in behaviour, preferences and goals, without having an impact on the organism's [[phenotype]]. Popper postulates that such purely behavioural changes are less likely to be lethal for the organism compared to drastic changes of the phenotype.<ref>K. Popper: ''Objective Knowledge'', section "Evolution and the Tree of Knowledge", subsection "Addendum. The Hopeful Behavioural Monster" (p. 281)</ref>

Popper contrasts his views with the notion of the "hopeful monster" that has large phenotype mutations and calls it the "hopeful behavioural monster". After behaviour has changed radically, small but quick changes of the phenotype follow to make the organism fitter to its changed goals. This way it looks as if the phenotype were changing guided by some invisible hand, while it is merely natural selection working in combination with the new behaviour. For example, according to this hypothesis, the eating habits of the giraffe must have changed before its elongated neck evolved. Popper contrasted this view as "evolution from within" or "active Darwinism" (the organism actively trying to discover new ways of life and being on a quest for conquering new ecological niches),<ref>{{Cite web |date=2 October 1986 |title=Philosophical confusion? |url=http://www.science-frontiers.com/philosophical-confusion.htm |url-status=dead |archive-url=https://web.archive.org/web/20140812204024/http://www.science-frontiers.com/philosophical-confusion.htm |archive-date=12 August 2014 |access-date=12 August 2014 |publisher=Science-Frontiers.com}}</ref><ref>Michel Ter Hark: ''Popper, Otto Selz and the Rise Of Evolutionary Epistemology'', pp. 184 ff</ref> with the naturalistic "evolution from without" (which has the picture of a hostile environment only trying to kill the mostly passive organism, or perhaps segregate some of its groups).

Popper was a key figure encouraging patent lawyer [[Günter Wächtershäuser]] to publish his [[iron–sulfur world hypothesis]] on [[abiogenesis]] and his criticism of [[Primordial soup|"soup" theory]].

On the creation-evolution controversy, Popper initially wrote that he considered it
{{blockquote|a somewhat sensational clash between a brilliant scientific hypothesis concerning the history of the various species of animals and plants on earth, and an older metaphysical theory which, incidentally, happened to be part of an established religious belief}}
with a footnote to the effect that he
{{blockquote|agree[s] with Professor C.E. Raven when...he calls this conflict 'a storm in a Victorian tea-cup'...<ref>Karl R. Popper, ''The Poverty of Historicism'', p. 97</ref>}} In his later work, however, when he had developed his own "spearhead model" and "active Darwinism" theories, Popper revised this view and found some validity in the controversy:
{{blockquote|I have to confess that this cup of tea has become, after all, ''my'' cup of tea; and with it I have to eat humble pie.<ref>Section XVIII, chapter "Of Clouds and Clocks" of ''Objective Knowledge''.</ref>}}

==== Free will ====
Popper and [[John Eccles (neurophysiologist)|John Eccles]] speculated on the problem of [[free will]] for many years, generally agreeing on an [[Interactionist dualism|interactionist dualist]] theory of mind. However, although Popper was a body-mind dualist, he did not think that the mind is [[substance dualism|a substance separate from the body]]: he thought that mental or psychological properties or aspects of people [[property dualism|are distinct from physical ones]].<ref>Popper, K. R. "Of Clouds and Clocks," in his Objective Knowledge, corrected edition, pp. 206–255, Oxford, Oxford University Press (1973), p. 231 footnote 43, & p. 252; also Popper, K. R. ''[http://www.informationphilosopher.com/solutions/philosophers/popper/natural_selection_and_the_emergence_of_mind.html "Natural Selection and the Emergence of Mind"]'', 1977.</ref>

When he gave the second [[Arthur Holly Compton]] Memorial Lecture in 1965, Popper revisited the idea of [[quantum indeterminacy]] as a source of human freedom. Eccles had suggested that "critically poised neurons" might be influenced by the mind to assist in a decision. Popper criticised Compton's idea of amplified quantum events affecting the decision. He wrote:

{{blockquote|The idea that the only alternative to determinism is just sheer chance was taken over by [[Moritz Schlick|Schlick]], together with many of his views on the subject, from [[David Hume|Hume]], who asserted that "the removal" of what he called "physical necessity" must always result in "the same thing with ''chance''. As objects must either be conjoin'd or not,... 'tis impossible to admit of any medium betwixt chance and an absolute necessity".}}

{{blockquote|I shall later argue against this important doctrine according to which the alternative to determinism is sheer chance. Yet I must admit that the doctrine seems to hold good for the quantum-theoretical models which have been designed to explain, or at least to illustrate, the possibility of human freedom. This seems to be the reason why these models are so very unsatisfactory.<ref>Popper, K. R. "Of Clouds and Clocks," in: ''Objective Knowledge'', corrected edition, p. 227, Oxford, Oxford University Press (1973). Popper's Hume quote is from ''Treatise on Human Understanding'', (see note 8) Book I, Part I, Section XIV, p. 171</ref>}}

{{blockquote|Hume's and Schlick's ontological thesis that there cannot exist anything intermediate between chance and determinism seems to me not only highly dogmatic (not to say doctrinaire) but clearly absurd; and it is understandable only on the assumption that they believed in a complete determinism in which chance has no status except as a symptom of our ignorance.<ref>''Of Clouds and Clocks'', in ''Objective Knowledge: An Evolutionary Approach'', Oxford (1972) pp. 227 ff.</ref>}}

Popper called not for something between chance and necessity but for a combination of randomness and control to explain freedom, though not yet explicitly in two stages with random chance before the controlled decision, saying, "freedom is not just chance but, rather, the result of a subtle interplay between something almost random or haphazard, and something like a restrictive or selective control."<ref>''ibid'', p. 232</ref>

Then in his 1977 book with John Eccles, ''The Self and its Brain'', Popper finally formulates the two-stage model in a temporal sequence. And he compares free will to Darwinian evolution and natural selection:

{{blockquote|New ideas have a striking similarity to genetic mutations. Now, let us look for a moment at genetic mutations. Mutations are, it seems, brought about by quantum theoretical indeterminacy (including radiation effects). Accordingly, they are also probabilistic and not in themselves originally selected or adequate, but on them there subsequently operates natural selection which eliminates inappropriate mutations. Now we could conceive of a similar process with respect to new ideas and to free-will decisions, and similar things.}}

{{blockquote|That is to say, a range of possibilities is brought about by a probabilistic and quantum mechanically characterised set of proposals, as it were—of possibilities brought forward by the brain. On these there then operates a kind of selective procedure which eliminates those proposals and those possibilities which are not acceptable to the mind.<ref>Eccles, John C. and Karl Popper. ''The Self and Its Brain: An Argument for Interactionism,'' Routledge (1984)</ref>}}