See
part I: some history of science and logical empiricism,
part II: problems of logical empiricism, critical rationalism and its problems.
After the unsuccessful attempts to found science on common sense notions as seen in the programs of logical empiricism and critical rationalism, people looked for new ideas and explanations.
The Kuhnian View
Thomas Kuhn's enormously influential work on the history of science is called the Structure of Scientific Revolutions. He revised the idea that science is an incremental process accumulating more and more knowledge. Instead, he identified the following phases in the evolution of science:
prehistory: many schools of thought coexist and controversies are abundant,
history proper: one group of scientists establishes a new solution to an existing problem which opens the doors to further inquiry; a so called paradigm emerges,
paradigm based science: unity in the scientific community on what the fundamental questions and central methods are; generally a problem solving process within the boundaries of unchallenged rules (analogy to solving a Sudoku),
crisis: more and more anomalies and boundaries appear; questioning of established rules,
revolution: a new theory and weltbild takes over solving the anomalies and a new paradigm is born.
Another central concept is incommensurability, meaning that proponents of different paradigms cannot understand the other's point of view because they have diverging ideas and views of the world. In other words, every rule is part of a paradigm and there exist no trans-paradigmatic rules.
This implies that such revolutions are not rational processes governed by insights and reason. In the words of Max Planck (the founder of quantum mechanics; from his autobiography):
A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it.
Kuhn gives additional blows to a commonsensical foundation of science with the help of Norwood Hanson and Willard Van Orman Quine:
every human observation of reality contains an a priori theoretical framework,
underdetermination of belief by evidence: any evidence collected for a specific claim is logically consistent with the falsity of the claim,
every experiment is based on auxiliary hypotheses (initial conditions, proper functioning of apparatus, experimental setup,...).
People slowly started to realize that there are serious consequences in Kuhn's ideas and the problems faced by the logical empiricists and critical rationalists in establishing a sound logical and empirical foundation of science:
postmodernism,
constructivism or the scoiology of science,
relativism.
Postmodernism
Modernism describes the development of Western industrialized society since the beginning of the 19th Century. A central idea was that there exist objective true beliefs and that progression is always linear.
Postmodernism replaces these notions with the belief that many different opinions and forms can coexist and all find acceptance. Core ideas are diversity, differences and intermingling. In the 1970s it is seen to enter scientific and cultural thinking.
Postmodernism has taken a bad rap from scientists after the so called Sokal affair, where physicist Alan Sokal got a nonsensical paper published in the journal of postmodern cultural studies, by flattering the editors ideology with nonsense that sounds good.
Postmodernims has been associated with scepticism and solipsism, next to relativism and constructivism.
Notable scientists identifiable as postmodernists are Thomas Kuhn, David Bohm and many figures in the 20th century philosophy of mathematics. As well as Paul Feyerabend, an influential philosopher of science.
Constructivism
To quote the Nobel laureate Steven Weinberg on Kuhnian revolutions:
If the transition from one paradigm to another cannot be judged by any external standard, then perhaps it is culture rather than nature that dictates the content of scientific theories.
Constructivism excludes objectivism and rationality by postulating that beliefs are always subject to a person's cultural and theological embedding and inherent idiosyncrasies. It also goes under the label of the sociology of science.
In the words of Paul Boghossian (in his book Fear of Knowledge: Against Relativism and Constructivism):
Constructivism about rational explanation: it is never possible to explain why we believe what we believe solely on the basis of our exposure to the relevant evidence; our contingent needs and interests must also be invoked.
The proponents of constructivism go further:
[...] all beliefs are on a par with one another with respect to the causes of their credibility. It is not that all beliefs are equally true or equally false, but that regardless of truth and falsity the fact of their credibility is to be seen as equally problematic.
From Barry Barnes' and David Bloor's Relativism, Rationalism and the Sociology of Knowledge.
In its radical version, constructivism fully abandons objectivism:
Objectivity is the illusion that observations are made without an observer (from the physicist Heinz von Foerster; my translation)
Modern physics has conquered domains that display an ontology that cannot be coherently captured or understood by human reasoning (from the philosopher Ernst von Glasersfeld); my translation
In addition, radical constructivism proposes that perception never yields an image of reality but is always a construction of sensory input and the memory capacity of an individual. An analogy would be the submarine captain who has to rely on instruments to indirectly gain knowledge from the outside world. Radical constructivists are motivated by modern insights gained by neurobiology.
Historically, Immanuel Kant can be understood as the founder of constructivism. On a side note, the bishop George Berkeley went even as far as to deny the existence of an external material reality altogether. Only ideas and thought are real.
Relativism
Another consequence of the foundations of science lacking commonsensical elements and the ideas of constructivism can be seen in the notion of relativism. If rationality is a function of our contingent and pragmatic reasons, then it can be rational for a group A to believe P, while at the same time it is rational for group B to believe in negation of P.
Although, as a philosophical idea, relativism goes back to the Greek Protagoras, its implications are unsettling for the Western mid: anything goes (as Paul Feyerabend characterizes his idea of scientific anarchy). If there is no objective truth, no absolute values, nothing universal, then a great many of humanity's century old concepts and beliefs are in danger.
It should however also be mentioned, that relativism is prevalent in Eastern thought systems, and as an example found in many Indian religions. In a similar vein, pantheism and holism are notions which are much more compatible with Eastern thought systems than Western ones.
Furthermore, John Stuart Mill's arguments for liberalism appear to also work well as arguments for relativism:
fallibility of people's opinions,
opinions that are thought to be wrong can contain partial truths,
accepted views, if not challenged, can lead to dogmas,
the significance and meaning of accepted opinions can be lost in time.
From his book On Liberty.
Epilogue
But could relativism be possibly true? Consider the following hints:
Epistemological
problems with perception: synaesthesia, altered states of consciousness (spontaneous, mystical experiences and drug induced),
psychopathology describes a frightening amount of defects in the perception of reality and ones self,
people suffering from psychosis or schizophrenia can experience a radically different reality,
free will and neuroscience,
synthetic happiness,
cognitive biases.
Ontological
nonlocal foundation of quantum reality: entanglement, delayed choice experiment,
illogical foundation of reality: wave-particle duality, superpositions, uncertainty, intrinsic probabilistic nature, time dilation (special relativity), observer/measurment problem in quantum theory,
discreteness of reality: quanta of energy and matter, constant speed of light,
nature of time: not present in fundamental theories of quantum gravity, symmetrical,
arrow of time: why was the initial state of the universe very low in entropy?
emergence, selforganization and structureformation.
In essence, perception doesn't necessarily say much about the world around us. Consciousness can fabricate reality. This makes it hard to be rational. Reality is a really bizarre place. Objectivity doesn't seem to play a big role.
And what about the human mind? Is this at least a paradox free realm? Unfortunately not. Even what appears as a consistent and logical formal thought system, i.e., mathematics, can be plagued by fundamental problems. Kurt Gödel proved that in every consistent non-contradictory system of mathematical axioms (leading to elementary arithmetic of whole numbers), there exist statements which cannot be proven or disproved in the system. So logical axiomatic systems are incomplete.
As an example Bertrand Russell encountered the following paradox: let R be the set of all sets that do not contain themselves as members. Is R an element of itself or not?
If you really accede to the idea that reality and the perception of reality by the human mind are very problematic concepts, then the next puzzles are:
why has science been so fantastically successful at describing reality?
why is science producing amazing technology at breakneck speed?
why is our macroscopic, classical level of reality so well behaved and appears so normal although it is based on quantum weirdness?
are all beliefs justified given the believers biography and brain chemistry?
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