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Is there any new, i.e., conceived in the last century, philosophical idea that radically changed the world in the twentieth century? I know it's kinda hard to come up with a precise and definitive answer because honestly we have virtually no historical distance but I'd love to give some thought to the matter anyway.

By changing the world I mean an idea capable of causing people to act and live differently and by people I don't mean single isolated individuals, I'm talking about changes on a massive scale. Enlightenment philosophers or Karl Marx seem to me good examples of philosophers changing the world. Besides philosophy, one might very well mention for instance the World Wide Web, the nation-state, book printing, electricity (which isn't an idea but a general term including several phenomena but you know what I mean) and just dozens of others. However, just so we're clear, I'm asking only for philosophical ideas, let me repeat that once again.

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    You may be interested in Badiou's The Century -- he notes that the last century was "haunted" by the idea of creating "a new man" – Joseph Weissman Aug 13 '11 at 13:42
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    This isn't really a philosophical question; it is a historical question. – Michael Dorfman Aug 14 '11 at 9:44
  • If you take free content as a philosophy, it has changed the world to a large degree. From google to facebook, the internet has dominated everything through the simple philosophy of being free to end-users. – apoorv020 Aug 17 '11 at 15:03
  • Would the idea of "artificial intelligence" qualify? – Doug Spoonwood Sep 27 '11 at 13:32
  • Even if it did, I don't think anyone would agree with that assertion that AI radically changed the world (yet). Strong AI, yes, but it hasn't been invented yet, and anything below that ("that" being near sentience or better) is just fancy programming, and better classified under "circuit logic manipulation" or something. – stoicfury Sep 29 '11 at 13:09

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Bertrand Russel's paradox overturned the 19th century ideas of how to ground mathematics and set theory, arguably setting the stage for Godel's incompleteness theorem, and giving a very deep new appreciation for unknowability (both directly and thanks to Godel). I count these as both mathematical and philosophical ideas, given how important they are for epistemology.

The idea of relativism, though difficult for me to trace, is now dominant in social and political sciences, and I think has had a huge impact on modern life, partly positive (tolerance) and partly negative (opinion trumps research). I am not sure whether it gained favor as fallout from Russel's paradox and Wittgenstein, but it seems related in spirit in some ways.

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    Russell's paradox also made possible a mathematical logic, and hence computers, and hence SE. How much more radical can you get? – user523 Aug 13 '11 at 21:26
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    @TimLymington Russell's paradox was not needed in the least bit for computers. Computers' logic computation comes essentially from Boolean algebra. – Evpok Aug 16 '11 at 22:32
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    @Evpok: yes, and without Principia Mathematica Boolean algebra would have remained an interesting abstract exercise. – user523 Aug 17 '11 at 10:17
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    @TimLymington: The intellectual thread that led to modern electronic computers being analyzed and designed using boolean algebra was primarily through Claude Shannon, whose thesis referenced Couturat and Whitehead's books, before Russell/Principia . – Mitch Sep 27 '11 at 3:05
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    @TimLymington 1. Principia Mathematica was an abstract exercise. 2. Boolean Algebra has connections with classical propositional logic. And you can even find Boole applying his own algebra (which isn't quite our abstract algebra) to the analysis of arguments. If you were to claim that without mathematical logic Boolean Algebra would have remained an abstract exercise that might work. But, plenty of other significant works here exist than Principia Mathematica. – Doug Spoonwood Sep 27 '11 at 13:47
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Marxist theory was developed in the 19th century by two German philosophers, Karl Marx and Friedrich Engels.

Those philosophical ideas indirectly led to many world-changing events in the twentieth century:

The theory led more or less directly to the Marxism-Leninism political movement, Maoism, etc. which later led to events in the twentieth century that changed the world: the 1917 October Revolution, the Union of Soviet Socialist Republics (1922 - 1991), McCarthyism, The Korean War, The Vietnam War, the Cold War (roughly 1946 to 1991), etc.

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    The OP explains (in the first sentence) that he's looking for ideas from the twentieth century. – Mitch Sep 28 '11 at 18:07
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    Oops. I guess I still think of "the last century" as the 1800s. – David Cary Sep 29 '11 at 1:23
  • Fascism however, belongs to the last century. Pretty rickety on the philosophical underpinnings though. – CriglCragl Jan 30 '18 at 23:53
  • Also, it is highly debatable whether Marxism is philosophy. It appears to be politics, sociology and economics informed by some convenient philosophical dogmas. I'd say this is why it had such a profound effect, that it was not a philosophy. . . . – PeterJ Feb 1 '18 at 11:35
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There's and old joke with a lot of truth in it that once a piece of philosophy becomes well understood or important people stop calling it "philosophy" and start calling it mathematics or give it its own subject name.

In particular I would nominate Computational Complexity as a philosophical idea that has changed the world in the last century, even though these days it would be called "mathematics" or "theoretical computer science". The idea that some computations can be proved to take impossibly long to do now underlies all computer security systems, meaning that trade can happen securely online.

  • Though I agree with you about your first statement, few people would recognize a mathematical idea as a kind of philosophical idea. That is, culturally, this is an inappropriate answer to the question. (and if you were to allow mathematical ideas, I'd figure that there are other concepts that had much more practical changes than computational complexity). – Mitch Sep 28 '11 at 18:04
  • True indeed. This field is a nursery for potent thoughts. It's just that no one traces back this far for all material things that had an impact. – user2411 May 7 '13 at 18:38
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Given the nature of philosophy, i.e. how it is viewed by the general public as "wishy-washy" or "there are no answers in philosophy", and the quarrels of philosophers among themselves (both affecting the rate at which philosophical ideas get accepted by the layperson), I can't really imagine a theory not just in the twentieth century but in the entire history of philosophy that radically changed the world at the magnitude you seem to be asking for.

One might argue that the move from scientists generally adhering to dualism-like theories to physicalist-like theories gave rise to whole fields of science where people could now access the human mind through analysis of the physical matter (psychology, sociology, neuroscience, etc) which in turn radically changed our understanding of how people are, both in normal circumstances and when affected by psychological issues (no more demons/witches/etc).

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    Even it's that true, it's not the OP's question. I find it difficult to see how relativity, quantum mechanics, and black hole holography have radically changed the world, causing changes on a massive scale, not just the addition of a few extra chapters in a physics book. Personally, I think the question is unfair, because philosophical implications must logically underlie every theory in science, and therefore every advance. What's more, it's never just one, but a mountain of theories building upon each other. The answer is long and complex; I'll defer to @DBK on these issues. – stoicfury Apr 2 '12 at 14:05
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    The development of quantum mechanics created all modern technology, since it was required for understanding semi-conductor physics. – Ron Maimon Apr 2 '12 at 16:41
  • Your views suggest that you (or the people you mention) have a narrow view of philosophy. Philosophy need not be wishy-washy and it does produce answers but one would have to venture beyond the walls of the Academy to see this. That scientists and philosophers so often adhere to dualistic theories is their problem. If they do this then they cannot expect to solve any problems in philosophy. Mutter mutter. . . – PeterJ Feb 1 '18 at 11:40
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Game theory.

Historically, many people make choices in terms of "all else being equal, what is the rational choice: one more person -- me -- choosing to do X rather than choosing to do Y?".

Typically such choices, when considered in isolation, seem to have relatively little impact on the world.

Game theory (1928, John von Neumann, and later developed in the 1950s by John Nash) leads to thinking in terms of "Given that there are millions of other people in exactly the same situation I am in, what is the rational choice: all of us choosing to do X, or all of us choosing to do Y?".

Millions of people, doing relatively little work each, can have huge effects on the world -- building entire cities, making entire species go extinct, planting entire forests, etc.

This has had an effect on how people act in a surprising variety of different situations, from wildlife conservation and environmental sustainability to the mechanism design used to set up eBay auctions to military missile defense strategies.

  • Game theory coupled with Chaos theory basically allowed a mathematical approach to ethics, responsibility, and free will in a way that had never been done before. – Ask About Monica Jan 31 '18 at 16:46
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The most significant idea of philosophy in the 20th century, by far, and the one that had incomparable revolutionary impact on the world is the idea of logical positivism. This consists of two parts:

  • Logical: using a formal language to describe what you mean exactly and precisely, like when you program a computer.
  • Positivism: making only statements about things that are subject to direct observation, or which can be reduced to these by a chain of definitions, so that you make sure you know what you are talking about.

The positivism is important, because it means that to give meaning to a question, you must reduce the terms to simpler definitions, until you reach either bedrock mathematical terms, like integers, or terms that can be defined by simple experience, like "I see a white circle".

The point of this exercise is that many superficially sensible questions are actually meaningless:

  • If the universe had a beginning, doesn't it require a cause?
  • If you will to move your arm, do your atoms determine the motion, or does your will?
  • What is gravity made of?

These types of questions are too vague to be meaningful, and when formulated properly, are shown to be mostly meaningless, or else the answer becomes obvious. The revolutionary effects of positivism meant that essentially all the questions of classical philosophy were either answered or mooted. It also allowed Turing to identify mind with computation , an identification which resolved many of the philosophy of mind difficulties.

Aside from the computer revolution, which was mostly through events in mathematical logic which have some overlap with this--- Russell and Whitehead are significant for Godel--- the most radical world-altering consequences of this philosophy was in physics, where it was the central motivating idea behind at least four separate revolutionary ideas (with more examples if you go back further, and as many as you like if you go to less Earth shattering examples).

  • Relativity
  • Quantum Mechanics
  • S-matrix theory (aka string theory)
  • Holographic principle

There are more: here and here. Most of the questions non-physicists ask are very ignorant, because they do not have a training in positivism, so that they ask vague or meaningless questions.

Once you have a little training in positivism (it doesn't take long), the discourse of non-positivists, which includes nearly all contemporary philosophers, becomes worthless and pointless to read, since non-positivists do not know how to identify questions with one another, and so spend most of the time arguing about positivistically equivalent positions, or inconsequential meaningless things.

Relativity (1900s)

In the 19th century, people believed in a lumineferous ether, a medium of light. Einstein showed that you could not measure your speed with respect to the ether, and therefore it is superfluous. Using positivism, he concludes that the ether is an unobservable entity, and therefore is meaningless. The ether was disproved in this way, the major step was positivism.

Ernst Mach developed philosophical positivism, along with important work in physics (the Mach number is named after him, since he studied supersonic fluid flow). Einstein admired and respected Mach, and made bold use of positivism in the early years.

Quantum Mechanics (1920s)

In quantum mechanics, positivism is the most important ingredient. The act of defining a measuring device and a measurement as a separate aspect of the theory from the time evolution was due to the explicitly positivist outlook of Bohr and Heisenberg.

The major idea of making a matrix description of the electron came from renouncing the idea of a microscopic well defined orbit, in favor of quantities which determine the atomic transitions, which are all that we can observe. As Pauli and Heisenberg argued, if you can't determine where the electron is when it is in the ground state of Hydrogen, on the left of the nucleus or on the right, even in principle (at least not without knocking it out of the ground state), then this question is meaningless, and the location of the electron in its orbit in the ground state is meaningless.

The positivism is still central to the interpretation of the most common statement of the uncertainty principle. Because we cannot measure the position and momentum simultaneously, these quantities do not exist simultaneously. The modern formalism does not include position and momentum, but a wavefunction which gives probability amplitudes for either.

S-matrix theory (1950s-1960s)

In the 1960s, physicists made the most radical positivist proposal you can imagine. The idea here was to renounce everything except for incoming and outgoing states in a scattering experiment, so that a physicist cannot describe events in space and time inbetween.

To make this clear: the S-matrix practitioners were saying that space is a reconstructed illusion, time is a reconstructed illusion (inasmuch as these statements are meaningful positivistically or mathematically), the only thing that is full meaningful to talk about are free cold particles in the infinite past, free cold particles in the infinite future, and an S-matrix to connect them.

This idea is manifestly absurd! It is the extreme of positivism. The S-matrix theory is inducting a principle: it is hard to make local measurements at a point in quantum field theory, and it is easy to do scattering experiments, so the idea that scattering is fundamental, and not space-time points. But this idea is completely crazy--- it means that fundamentally you can't talk about your own feet, or your grocery store, only about a large number of cold particles that come in to form the Earth and the Sun and everything else in the infinite past, and free cold particles in the infinite future, and an S-matrix that connects them (in one step).

This idea is so absurd, I could not accept it when I was a kid. I rebelled and rebelled against it, and I struggled to understand how sane people could believe this. Sidney Coleman explained it to me as follows:

Imagine you have some experiment that is not an S-matrix experiment. Imagine a robot doing the experiment. Assemble such a robot and some Hydrogen bombs from cold asymptotic particles, let the robot do the experiment, beam the results out in a photon beam to infinity, the detonate the H-bombs, so that the whole thing is blown into cold asymptotic particles. The S-matrix for that process includes the result of the experiment.

This was persuasive to me that the S-matrix was philosophically enough, but I could not be persuaded it was mathematically enough--- how do you reconstruct space and time from S-matrix?

This idea is positivism to the point of nihilism--- all space and time are rejected. This was a bit too radical for Feynman, but Gell-Mann could accept it, and helped found it and nurture it. The developments in the 1960s led to Regge theory and string theory, and it was realized by 1974 that S-matrix theory, in its string theory culmination, could only be a theory of everything, when Schwarz and Scherk made string theory a theory of everything.

Holographic principle

Within string theory, and quantum gravity in general, the problem of black hole interiors was solved by a positivistic rejection of the black hole interior as separate from the exterior, since no observer can see both. The development of this principle refounded string theory, giving it a new founding principle, and at the same time explained the S-matrix theory, since the holographic description of space time turns into the S-matrix for flat spacetime with distant boundaries.

The holographic reconstruction of the interior from the boundary is the sensible way to understand how an S-matrix still encodes local physics. The idea is central to modern physics, in that the only theory we have of quantum gravity, is positivist to the point of nihilist--- it reconstructs everything from boundary data.

Rejection of positivism

Physicists are not immune to philosophical fasion, and between the years of 1974 and 1984, string theory was rejected essentially for the same reasons logical positivism was dying in other fields--- marijuana and New-Age thinking, plus a desire to revive the nice classical questions of philosophy about an objective world.

This is ridiculous reactionary nonsense, but it led to a decade of obscurity for string theory. In 1984, physicists picked up the theory, but not the positivism, nor the S-matrix origins. In the 1990s, the holographic principle gave new motivation to the S-matrix program, and in the last decade, S-matrix methods have been revived again in supergravity perturbation theory, and in studies of the pomeron by Polchinski, Tan, and collaborators.

This is a welcome relief--- positivism had almost been dealt the same blow that heliocentrism was dealt in Greek times--- death by stupidity.

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    I find it not too convincing that one should make a theory without the eather-not even as an instrument-but then accept the wave function. Both is okay, but if I use the latter, then even if I can't measure the eather, I could still make a theory with involving it. Aside from that I never understood the real difference of an eather and a field. What I also find interesting is your identification of S-matrix theory with string theory. At this point, I don't know what or how strings contain "everything else", but all the books I know introduce the picture which involve actual strings first. – Nikolaj-K Apr 2 '12 at 8:33
  • And although I like S-matrices, I don't know how can compute anthing from them if one doesn't sum up matrix elements to form some incomming bunch of things (living in space or momentum space!) or if one abstracts enought to compute observables in statistical mechanics. – Nikolaj-K Apr 2 '12 at 8:58
  • @NickKidman: yes, exactly, this was my problem with the idea, but I was wrong. But it is possible to reinterpret S-matrix theory as a holographic idea, and yes, you do do some summing up over incoming wave-packets. The point is you don't talk about space-time localized anything. – Ron Maimon Apr 2 '12 at 16:42
  • reinterpret "as a holographic idea" in the sense that everything inside is also on the boundary and this is where the states for the S-Matrix are? And did the S-Matrix people know of this? – Nikolaj-K Apr 2 '12 at 17:42
  • @NickKidman: Everything inside is represented on the boundary, and only the boundary representation is mathematically self-consistent, local, or complete. The S-matrix people did not know this precisely, but their intuition regarding the primacy of the S-matrix is essentially mathematically equivalent, and the philosophy is a primitive version of holography. I always wondered how people could derive string theory, which is gravitational, without a gravitational principle underlying things. But S-matrix is gravitational holography, in a primitive stunted asymptotically flat form. – Ron Maimon Apr 2 '12 at 18:51
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I would posit that the naturalistic philosophies surrounding eugenics had a tremendous effect on the way we perceive human life. It was the way in which the Nazi sold mass murder to the German people.

The view that we should aid the evolutionary process seemed like a good endeavor to some. If we are all just the process of the mingling of the primordial soup then ensuring the strongest survive and weak perish seems reasonable.

If then the German government would institute a program of riding Europe of it more undesirable genetic specimens then so be it. As long as we are staying true to the maxim of might is right then we are doing no foul. We can then find to backing to rid our countries of all our inferiors specimens. Be they black, Jewish, Gypsies or the crippled.

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    The philosophy in question came from the 19th century, from Galton and the eugenicists, and also from the contemptible genocidal tradition started by Columbus. The political movement in the 19th century was colonial genocide, which differed from Nazi genocide only in scale, methods, familiarity of victims, and location. – Ron Maimon Jul 29 '14 at 8:37
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    Well it had its awe inspiring and terrifying conclusion in the 20th century. – Neil Meyer Jul 29 '14 at 10:13
  • @Ron Maimon - Ludwijk didn't ask when a particular philosophy originated. He asked about philosophies that had a major impact in the 20th century, regardless of when it was born. – David Blomstrom Jan 31 '18 at 2:15
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Alan Turing's conceptualization of modern computing was a philosophical thought experiment when he conceived it. But every computer-based technological advance since then has been founded on that thought-experiment. It's hard to imagine any other 20th century idea having as large a measurable impact to date.

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Ghandi's philosophy around non-violence directly and profoundly, and lastingly, changed the future of India.

The North Korean philosophy of Juche is continuing to influence the world.

Liberation Theology has had practical impacts on radicals, but I don't think that fits the OPQ

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    Not just India, America too. Martin Luther King said explicitly that he had been inspired by Gandhi. – Mozibur Ullah Feb 1 '18 at 0:10
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Yes, there does exist such a philosophical idea that radically changed the world in the twentieth century.

Radical (root) feminism with its idea of a transhistorical patriarchy that has men generally or universally either oppressing or suppressing or silencing women has lead to changes in law, politics, courtship and dating, marriage, education, sections of historical scholarship, theology, and culture.

Legalizing Misandry indicates how ideological feminism via its idea of a transhistorical patriarchy has affected family court law, how it more-or-less created sexual harassment law, domestic violence law (which alone imply the dating world as affected), and how it has affected academic discussions and scholarship. Family court law affects how people interact with each other after divorces, as well as affecting how interactions with children proceed. Domestic violence law affects how the police behave towards citizens. Sexual harassment law, highly influenced by the radical feminist Catherine Mackinnon, has affected not only how things work in academia, but also the workplace of countless organizations which influences workers in many, many ways.

Spreading Misandry argues that ideological feminism, which includes radical feminism, has influenced popular and elite culture. Parts of book lists such as this one probably also indicate the same.

Sanctifying Misandry shows how ideological feminism has affected theology. Theology affects how religious leaders go about their business, which in turn affects how communities of people organize themselves.

The War Against Boys by feminist Christian Hoff Sommers shows how feminism has affected education and even elementary school teaching, and it comes as hard to see how radical feminism with its notion of a transhistorical patriarchy doesn't come as an influencing factor here also.

Psychotherapy, via feminist therapy, if "feminist therapy" deserves the name therapy at all, consists of another realm of influence.

I haven't seen a word of feminist Daphnae Patai's Heterophobia, but the blurb at Amazon and Nathanson and Young's commentary in Legalizing Misandry drawing on Patai indicate that she indicates that the idea of a transhistorical patriarchy has affected several areas mentioned above.

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    Feminism is a nice example. But it works without the restriction to what you've called radical feminism (the idea that feminism must be misandrist); the simple idea that gender is not grounds for discrimination is a philosophical thesis which has had, and continues to have, huge implications. – dbmag9 May 3 '13 at 15:52
  • This answer looks like it could use diversification of the list of publications. As a counterpoint (but in a similar vein) to Doug's existing answers, I might suggest Naomi Wolfe's Fire with Fire, but perhaps someone can suggest more scholarly feminist works to be included in the list. – Niel de Beaudrap May 3 '13 at 18:03
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    @NieldeBeaudrap The books by Paul Nathanson and Katherine Young are scholarly. I also didn't make any claim about the idea of a transhistorical patriarchy as misandrist. – Doug Spoonwood May 4 '13 at 5:27
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    @DougSpoonwood I don't see that you can claim 'transhistorical patriarchy' as more philosophical a thesis than 'gender is not grounds for discrimination'. Both give rise to a social movement with more or less cohesion and impact, but the theses are philosophical. – dbmag9 May 4 '13 at 8:03
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    @NieldeBeaudrap If I had used those books to indicate the spread of misandry, then yes I may have well said it. However, I did NOT use those books in that way. I only mentioned their titles and indicated them as books which show how the idea of a transhistorical patriarchy has influenced things. Also, the books by Katherine Young and Paul Nathanson do not merely suppose the spread of misandry as orchestrated, they argue for it as orchestrated. On top of that, we never knew structural gender inequality as only a female problem. Male only conscription has existed... – Doug Spoonwood May 4 '13 at 16:16
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Possibly existentialism.

Existentialism was born in the 19th century, but one of its most famous advocates was the French philosopher Jean-Paul Sartre, who was a huge fan of Latin American revolutionary Che Guevara.

I'm not sure if El Che considered himself an existentialist, or if he's considered an existentialist by others, but he seems to fit the bill.

Existentialism would appear to be a powerful force contrary to both apathy and determinism (e.g. lack of free will). The latter are presumably embraced by the empire builders, who would be expected to either demonize or snub existentialism.

There are lots of questions to explore here, but I suspect existentialism had a bigger impact on the 20th century that is generally recognized.

You mentioned Karl Marx. Che Guevara was a Marxist who took the movement in a new direction; Cuba was as exciting an experiment as the Soviet Union, which was very different and collapsed decades ago. Cuba is still standing strong.

  • The question concerns something conceived in the last century, but you reference a time period before 100 years ago. – Doug Spoonwood Apr 7 '18 at 1:47

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