Editing Karl Popper (section)

=== Philosophy of science ===
{{See also|Falsifiability}}

==== Falsifiability and the problem of demarcation ====
Popper coined the term "critical rationalism" to describe his philosophy.{{refn|name=popperproposecriticalrationalism}} Popper rejected the empiricist view (following from Kant) that [[basic statement]]s are infallible; rather, according to Popper, they are descriptions in relation to a theoretical framework.{{sfn|Thornton|2018}}  Concerning the method of science, the term "critical rationalism" indicates his rejection of classical [[empiricism]], and the classical [[inductivism|observationalist-inductivist]] account of science that had grown out of it.{{refn|name=poppercriticsofVienna}} Popper argued strongly against the latter, holding that [[scientific theories]] are abstract in nature and can be tested only indirectly, by reference to their implications.{{sfn|Thornton|2018|loc=Sec. 4}} He also held that scientific theory, and human knowledge generally, is irreducibly conjectural or hypothetical, and is generated by the creative imagination to solve problems that have arisen in specific historico-cultural settings.

Logically, no number of positive outcomes at the level of experimental testing can confirm a scientific theory, but a single counterexample is logically decisive; it shows the theory, from which the implication is derived, to be false. Popper's account of the logical asymmetry between [[Verification theory|verification]] and [[falsifiability]] lies at the heart of his philosophy of science. It also inspired him to take falsifiability as his criterion of [[Demarcation problem|demarcation]] between [[metaphysics]] and science: a theory should be considered scientific if, and only if, it makes predictions that can be falsified. This led him to attack the claims of both [[psychoanalysis]] and contemporary [[Marxism]] to scientific status, on the basis that it is not possible to falsify the predictions that they make.

To say that a given statement (e.g., the statement of a law of some scientific theory)—call it "T"—is "[[falsifiable]]" does not mean that "T" is false. It means only that the background knowledge about existing technologies, which exists before and independently of the theory, allows the imagination or conceptualization of observations that are in contradiction with the theory. It is only required that these contradictory observations can potentially be observed with existing technologies—the observations must be inter-subjective. This is the material requirement of falsifiability.  Alan Chalmers gives "The brick fell upward when released" as an example of an imaginary observation that shows that Newton's law of gravitation is falsifiable.{{sfn|Chalmers|2013|p=62}}

In ''All Life is Problem Solving'', Popper sought to explain the apparent progress of scientific knowledge—that is, how it is that our understanding of the universe seems to improve over time. This problem arises from his position that the truth content of our theories, even the best of them, cannot be verified by scientific testing, but can only be falsified.  With only falsifications being possible logically, how can we explain the [[growth of knowledge]]? In Popper's view, the advance of scientific knowledge is an ''evolutionary'' process characterised by his formula:{{sfn|Popper|1994|pp=2–3}}{{sfn|De Bruin|2006}}

<math>\mathrm{PS}_1 \rightarrow \mathrm{TT}_1 \rightarrow \mathrm{EE}_1 \rightarrow \mathrm{PS}_2. \, </math>

In response to a given problem situation (<math>\mathrm{PS}_1</math>), a number of competing conjectures, or tentative theories (<math>\mathrm{TT}</math>), are systematically subjected to the most rigorous attempts at falsification possible. This process, error elimination (<math>\mathrm{EE}</math>), performs a similar function for science that [[natural selection]] performs for [[biological evolution]]. Theories that better survive the process of refutation are not more true, but rather, more "fit"—in other words, more applicable to the problem situation at hand (<math>\mathrm{PS}_1</math>). Consequently, just as a species' biological fitness does not ensure continued survival, neither does rigorous testing protect a scientific theory from refutation in the future. Yet, as it appears that the engine of biological evolution has, over many generations, produced adaptive traits equipped to deal with more and more complex problems of survival, likewise, the evolution of theories through the scientific method may, in Popper's view, reflect a certain type of progress: toward more and more interesting problems (<math>\mathrm{PS}_2</math>). For Popper, it is in the interplay between the tentative theories (conjectures) and error elimination (refutation) that scientific knowledge advances toward greater and greater problems; in a process very much akin to the interplay between genetic variation and natural selection.

Popper also wrote extensively against the famous [[Copenhagen interpretation]] of [[quantum mechanics]]. He strongly disagreed with [[Niels Bohr]]'s [[instrumentalism]] and supported [[Albert Einstein]]'s [[scientific realist]] approach to scientific theories about the universe. He found that Bohr's interpretation introduced subjectivity into physics, claiming later in his life that: {{blockquote|Bohr was "a marvelous physicist, one of the greatest of all time, but he was a miserable philosopher, and one couldn't talk to him. He was talking all the time, allowing practically only one or two words to you and then at once cutting in."<ref>{{Cite news |last=Horgan |first=John |date=2018-08-22 |title=The Paradox of Karl Popper |work=[[Scientific American]] |url=https://blogs.scientificamerican.com/cross-check/the-paradox-of-karl-popper/ |url-access=subscription |access-date=2023-03-12}}</ref>}}This Popper's falsifiability resembles [[Charles Sanders Peirce|Charles Peirce]]'s nineteenth-century [[fallibilism]]. In ''Of Clocks and Clouds'' (1966), Popper remarked that he wished he had known of Peirce's work earlier.

==== Falsification and the problem of induction ====
Among his contributions to philosophy is his claim to have solved the philosophical [[problem of induction]]. He states that while there is no way to prove that the sun will rise, it is possible to formulate the theory that every day the sun will rise; if it does not rise on some particular day, the theory will be falsified and will have to be replaced by a different one. Until that day, there is no need to reject the assumption that the theory is true. Nor is it rational according to Popper to make instead the more complex assumption that the sun will rise until a given day, but will stop doing so the day after, or similar statements with additional conditions. Such a theory would be true with higher probability because it cannot be attacked so easily:
* to falsify the first one, it is sufficient to find that the sun has stopped rising;
* to falsify the second one, one additionally needs the assumption that the given day has not yet been reached.
Popper held that it is the least likely, or most easily falsifiable, or simplest theory (attributes which he identified as all the same thing) that explains known facts that one should rationally prefer. His opposition to positivism, which held that it is the theory most likely to be true that one should prefer, here becomes very apparent. It is impossible, Popper argues, to ensure a theory to be true; it is more important that its falsity can be detected as easily as possible.

Popper agreed with [[David Hume]] that there is often a psychological belief that the sun will rise tomorrow and that there is no logical justification for the supposition that it will, simply because it always has in the past. Popper writes,
{{blockquote|I approached the problem of induction through Hume. Hume, I felt, was perfectly right in pointing out that induction cannot be logically justified.{{sfn|Popper|1962|p=[https://archive.org/details/conjecturesrefut0000popp/page/42 42]}}}}