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The Structure Of Scientific Revolutions by Thomas Kuhn.
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The Meaning of “Structure” and “Revolution”

Many people have the idea that science progresses in a linear fashion—a straightforward march from ignorance to knowledge. If that were true, then there would be no such thing as scientific revolutions; old ideas would never need to be overturned, only built upon.

However, not only do scientific revolutions happen, they follow a specific pattern. The Structure of Scientific Revolutions lays out this pattern clearly and labels each part of it. The parts are:

  1. Normal science: The process of adding to existing knowledge through experiments and study.
  2. Puzzle-solving: Finding and fixing discrepancies in science as we currently understand it.
  3. Paradigm: This is a difficult word to define, and the author himself used it in many different ways. However, a paradigm could be seen as a perfect example of an idea, which can be expanded into the framework of a whole field of scientific understanding. For example, think about the evolution of humans from a more ape-like ancestor. Using that paradigm, evolutionary biologists form the framework for all of their studies.
  4. Anomaly: Someone finds a discrepancy between the current paradigm and experimental results.
  5. Crisis: The discrepancy can’t be resolved. Scientists loyal to the current paradigm try to dismiss it, while others try to show that the current paradigm is wrong.
  6. Revolution: The old paradigm is overthrown by new information, and eventually a new one takes its place—a “paradigm shift.”

There have been many such revolutions in history that completely changed the way people understood and approached the world. There was a mathematical revolution when the Greeks created proofs to show not only that certain mathematical formulae and postulates work, but why they work. Classical science had a revolution when laboratories and experiments became the preferred way of examining the world, a practice that may have begun with Galileo.

In the 19th century, entire fields of science were codified and sorted into paradigms, including heat, light, and electricity. Phenomena that had baffled scientists could now be categorized and understood. This was around the time of the Industrial Revolution, and likely a direct cause of it. There have been more since then, including Einstein’s theory of relativity and Planck’s first steps into quantum theory.

Revolutions happen because science is not a straightforward path toward what’s “true.” It’s more like a path away from what’s wrong.

Karl Popper’s work could be seen as the precursor to this concept of scientific revolutions. Popper taught that scientists come up with broad, testable ideas, and almost inevitably prove them wrong. Then they refine their ideas based on the new information and try again. This cycle of conjectures and refutations is similar to the idea of scientific revolutions, just on a much smaller scale.

“Normal Science” Within a Paradigm

The everyday work of scientists could be called normal science. This is when they work within a...

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The Structure Of Scientific Revolutions Summary Chapter 1: An Introduction to Structure

Our current view of science is pulled mostly from textbooks and similar sources, the formal education we receive in school. However, these sources are like guidebooks or pamphlets for tourists, rather than accurate pictures of scientific history. As a result, some of our ideas about science are wrong.

Many of those books imply that “science” is only the theories and discoveries in the books themselves, rather than a huge assortment of different fields, ideas, and practices. Also, they often focus on experimental science (the so-called Scientific Method), but ignore the importance of connecting the results of those experiments to more general theories.

If our current ideas about science are true, then science is just the complete collection of knowledge we have about the world. In that case, scientific historians should be devoted to figuring out who made which contributions to that collection—and, on the other hand, figuring out how and why so many mistakes and myths were wrongly added to it over the years.

However, recently, people are finding that scientific history isn’t a straight line from ignorance to knowledge like they thought. There are a few anomalies that...

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The Structure Of Scientific Revolutions Summary Chapters 2-4: All About “Normal Science”

Normal science means study based on previous scientific knowledge. That knowledge is usually learned through textbooks and formal education, which outline the main questions of a field and the methods used to answer them. Before textbooks became common, the old classics like Aristotle’s Physica had much the same purpose.

These books became influential for two reasons: first, because they were remarkable enough to attract a loyal following at a time when many schools of thought were competing with each other; second, because they left a lot of questions and unsolved problems—puzzles for their followers to solve.

Scientific discoveries that have those two characteristics become paradigms: They are perfect examples of their fields, and a lot of information can be extrapolated from them. Paradigms are at the heart of normal science. They are the models that future experiments and theories are based on. This is why we use phrases like Newtonian physics: physics based on the paradigm of Newton’s discoveries.

Students prepare for their work in chosen scientific communities by studying paradigms. Physicists, for example, would study Newton’s work and how to apply his...

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The Structure Of Scientific Revolutions Summary Chapters 5-6: Paradigms, Anomalies, and Discoveries

While historians can generally find the paradigms of a given community fairly easily, finding the specific rules that community followed is often much harder. For one thing, scientists may follow the same paradigm, but disagree on how to interpret it. Also, like professionals in any field, scientists have experience and knowledge that they either don’t communicate clearly or don’t realize others lack. Therefore, historians sometimes don’t recognize or understand the rules of the time because they’re couched in unfamiliar terms and assumptions.

Finally, just because there is a paradigm doesn’t mean that there is necessarily a set of rules to go with it. Sometimes historians are looking for something that simply isn’t there. However, even when paradigms lack specific rules, they can still restrict the scientific field by guiding the work.

To illustrate this point, consider a question asked by the philosopher Ludwig Wittgenstein. He wondered how we know to apply terms like “chair,” or “game,” to something we’ve never seen before.

The brief version of the answer is that we apply those terms to things that resemble other things we know by those names. For example, if...

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The Structure Of Scientific Revolutions Summary Chapters 7-9: How Crises Lead to Revolutions

If recognizing anomalies is a key part of new discoveries, it shouldn’t be surprising that it’s also part of inventing theories—remember that discovering new facts and developing new theories are closely related processes. That whole process is key to scientific revolutions.

This isn’t to say that normal science never solves anomalies—in fact, that’s one of its major functions. This is part of the reason paradigms are so hard to change: Scientists often assure themselves that there is a solution within the paradigm, it just hasn’t been found yet. Of course, this isn’t always the case.

On the very rare occasions that a field answers every question posed, such as the paradigm that has light traveling in the form of rays, that field stops being science and becomes an engineering tool. However, when those anomalies last long enough or reject some key part of the paradigm, it could be said that the fields they pertain to are in crisis.

In other words, an anomaly must seem like something more than a regular puzzle of normal science. Scientists in those fields have a growing awareness that something is fundamentally wrong, that nature is in direct conflict with their...

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Shortform Exercise: Recognize Crises

Not just the sciences, but almost every field has certain established beliefs and paradigms. This means that just about anything could be subject to a crisis.

Are there currently competing paradigms in your field of interest? The field doesn’t have to be in crisis for this to be relevant, just consider whether there are other schools of thought fighting to be recognized.

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The Structure Of Scientific Revolutions Summary Chapter 10-11: Changing Paradigms, Changing World

When paradigms change, it could be said that the entire world changes with them. Scientists ask different questions, follow different rules, and even use different terms (or use the same terms differently) after a paradigm change. Perhaps even more importantly, they see new information in studies they’ve done before.

This process resembles a gestalt shift. In psychology, a gestalt shift is when your perception of something suddenly changes—like a picture that you can see as either a rabbit or a duck (but never both at once).

However, there are important differences between a simple gestalt shift and a scientific revolution. Gestalt shifts can go either way—from duck to rabbit or rabbit to duck, and back again. The subject can even learn to make the switch at will. On the other hand, scientific revolutions generally go one way, and they are irreversible. A scientist who now views the world through a Copernican paradigm can never go back to Ptolemy. A period when scientific perceptions can switch back and forth is, by definition, a crisis.

In a gestalt shift, such as with the picture which could be seen as a duck or a rabbit, either interpretation is correct—or at...

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Shortform Exercise: Think About Revolutions

While Structure is, of course, focused on scientific revolutions, many discoveries and events across all fields have been described as revolutionary.

Think about a field that’s particularly interesting to you, whether scientific or otherwise. What’s something that has revolutionized that field?

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The Structure Of Scientific Revolutions Summary Chapter 12: Verifying a New Paradigm

Any new paradigm starts with one or, at most, a few people. These people usually have two things in common. First, they have focused their work on the anomalies that led to the current crisis. Second, they are young, or at least new to the field, and so are less set in the current methods and rules than their colleagues.

The challenge then becomes convincing the rest of the scientific community to adopt their new paradigm. Therefore, the two (or more) paradigms must now be compared both against nature and against each other.

Positive Verification

It’s impossible to test a paradigm in every way, under every circumstance. Therefore, many scientists would say that convincing the community to adopt a new paradigm isn’t about proving beyond doubt that it’s correct; instead it’s about showing that, given the available evidence, it is more likely to be correct than the other options.

This is called positive verification, and it offers a couple of testing methods. One way is to compare the paradigm or paradigms in question against all conceivable ones that explain the same data. Another is to come up with tests that the paradigm might reasonably be expected to...

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The Structure Of Scientific Revolutions Summary Chapter 13: The Nature of Scientific Progress

Most of the preceding arguments have been about progress: the nature of it, why it’s expected, and how it happens. However, “progress” is often a label put on events by their observers. The reason normal science seems to progress is clear: Scientists working within a paradigm have a clear set of problems to solve, and the means by which to solve them.

Normal science benefits from being fairly well insulated from the laypeople. Scientists generally only show their work to other scientists, meaning they can take certain beliefs and rules for granted. Furthermore, the scientists can choose problems they are reasonably confident they’ll be able to solve, without worrying about what needs to be solved, as an engineer or doctor might have to. All of this makes progress both faster and clearer than in other professions.

Do Other Fields Experience “Progress”?

Some may wonder why unscientific fields such as art and philosophy don’t seem to progress like people assume science does. However, this question may be fundamentally flawed, and the progress of other fields may be closer to science than people believe.

As an interesting example, painting was seen as a...

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The Structure Of Scientific Revolutions Summary Introductory Essay by Ian Hacking

(Shortform note: This essay is included as a preface to Structure in the 2012 edition, but its author suggests saving it until after you’ve read Kuhn’s book.)

This introductory essay by Ian Hacking discusses how and why The Structure of Scientific Revolutions was written, its impact on the scientific community, and whether it’s still relevant today (Shortform note: The first edition of Structure was published in 1962).

The sciences have changed a lot since Structure was first published. In 1962, physics was in the spotlight. It was the middle of the Cold War, so everyone had nuclear weapons on their minds. In fact, Kuhn himself was trained as a physicist.

Today, however, biotechnology is the hot field of science. On top of that, modern technology like computer simulations and the internet has changed how science is practiced and how ideas are shared.

Another major change since Structure was written has to do with fundamental physics itself. In 1962, physics had two major competing models of the universe: steady state and the Big Bang theory. By 2012, only the Big Bang...

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