Thomas Kuhn was a philosopher of science whose signature work was The Structure of Scientific Revolutions, published in 1962. One thing that book did was introduce the phrase “paradigm shift,” which became annoyingly overused (but thankfully, I don’t hear it quite as often recently, even in those language-mangling arenas, corporate conference rooms). The other thing Kuhn’s book did was change everybody’s conception about the way scientific (and other) understanding proceeds.

The idea current before The Structure of Scientific Revolutions was that scientific understanding progresses in a sort of continuous line. New observations and discoveries gradually show problems in older conceptual models, and new ideas and theories are developed that are more accurate representations of reality. Not so, observed Kuhn. Scientific knowledge isn’t simply a set of objective measurements and completely rational analysis by savants. It’s also enmeshed in social conventions and activity. Scientists, as well as interested lay people, share a consensus about a particular set of idea — a world view, or paradigm, in Kuhn’s parlance — and the replacement of one paradigm with another is also a social event.

One of Kuhn’s examples of how a paradigm shift works was the Copernican Revolution. It’s called a “revolution” not because there was fighting in the streets, but partly because Nicolaus Copernicus published a book in 1543 called On the Revolutions of the Heavenly Spheres. The book contained his argument that the Earth and the other planets revolved around the Sun, rather than the Earth being at the center. The old model had been around for more than a thousand years. It was associated with Claudius Ptolemy, a Roman Empire astronomer from the first century CE. Ptolemy lived in Egypt, which at the time was ruled by Rome. He might have been born in Greece, since his name is Greek, or Egypt — but there’s no real evidence either way. There were Egyptian pharaohs named Ptolemy too, so who knows.

Anyway, the Ptolemaic system of astronomy that put Earth at the center of everything was supported by some pretty complicated math enabling predictions of where to look in the night sky to see planets and stars. Ptolemy even formulated tables showing when solar and lunar eclipses were going to occur. He thought that universe was a set of nested spheres around the Earth, with the Sun and the various planets and stars each at their own level. He even computed the dimensions of the universe according to his model. And his predictions were pretty close; if you had access to a set of Ptolemy’s tables (and the requisite mathematical skill), you could figure out where to look to see Venus, or when an eclipse was going to occur.

Copernicus had a different idea; he thought the Sun was in the center, and the Earth, as well as the planets, rotated around it. Based on his observations, he was pretty sure the stars didn’t, though. What Kuhn noticed was that the paradigm Copernicus was suggesting did not necessarily fit the available data any better than the existing Ptolemaic paradigm. Ptolemy’s tables had been working for 1500 years, and they really did deliver reliable predictions, which is supposed to be the test of a scientific theory. The math involved was immensely more complex than anything required by Copernicus. If you plot the positions of the planets in the night sky over time, you’ll notice that they don’t move in anything like a simple line or arc; they loop around. Ptolemy accounted for this by some pretty amazing calculations. But Copernicus showed that you could account for exactly the same thing very simply if you just put the Sun at the center of things instead. But the Ptolemaic paradigm had been around for 1500 years and had the advantage of 1500 years of tweaking. Copernicus had a heliocentric model, but hadn’t done 1500 years’ worth of detail work, so his paradigm, at first, was worse at predicting the real world than the predominant paradigm.

Kuhn used that example to show that the adoption of the Copernican paradigm wasn’t because of careful consideration of how well the theory predicted real events, because at first it didn’t do as well. Instead, he said, the paradigm was adopted because of a change in the underlying way people thought of the universe itself, and that was influence less by data and observation than by their interactions with other people, and by thinking about what a new world view might offer in terms of explanatory power.

That was the problem with the Ptolemaic paradigm — it could account for the increasing accuracy of astronomical observations, but every time it had to be tweaked, it got more complicated. Looking at its future, the astronomers and mathematicians of the day saw nothing but a constant rise in that complexity. The Copernican, or heliocentric model had the promise of explaining future observations more simply. In Kuhn’s terms, it did a better job of accounting for anomalies.

Today is the anniversary of an event that sounds very strange in our modern era: the Condemnations of 1210-1277. The final condemnation was issued on March 7, 1277. So what on earth were the Condemnations? It all happened at the University of Paris, conducted by the Bishops of Paris. In the 1200s a university was quite a bit different from what we have today. It was a mixture of theological thought and the rudiments of science. In Medieval Europe, free inquiry and even observation were not encouraged. Quite the opposite, the conventional wisdom was that everything worth knowing was already known, and had been established either by Aristotle, in the realm of what passed for science, or by the Bible, in theology.

There were three sets of Condemnations, issued in 1210, 1270, and 1277. They forbid the reading of Aristotle’s books, refuted the philosophical tenets of Averroes (nobody remembers this guy; I’ll explain below), and in the final set in 1277, made some detailed points that are today fairly hard to follow because we lack the context.

Averroes was a philosopher from what is now Spain. At the time it was an Islamic state. His writings attracted adherents who came to be known as “Averroists,” who believed in the “unity of the intellect” that Averroes espoused — based on his study of Aristotle. We don’t need to get into the weeds here, but  the unity of the intellect was the idea that everybody shared the same intellect, or (sort of) the same mind. Averroes wasn’t an atheist, but Averroism was something of a synonym for atheism in Medieval times. Okay, though, enough about Averroes.

The Condemnations were pretty disorganized, and mixed theological topics up with philosophical ones (in those days “natural philosophy” was the closest they came to science). But here’s the basic idea of today’s episode: by listing a whole bunch of books and ideas that were suddenly off limits in the University of Paris (the biggest magnet school in Europe in the 1200s), they spurred a paradigm shift. If you were a philosopher or student, your old job had been to pore over Aristotle’s work, and related writings, to find answers to questions. The answers had to be there, because “everything worth knowing was already known.”

After the Condemnations, you had to start finding your own answers. And according to some historians (especially Pierre Duhem and Edward Grant), that turned out to spur the beginnings of scientific thought. And that gave us the opportunity to enjoy further scientific revolutions as paradigms began to shift left and right.

The title is from The Second Coming by William Butler Yeats. That poem also includes the familiar lines

“The best lack all conviction, while the worst
Are full of passionate intensity.”

Yeats wrote that almost exactly a century ago, in 1920. Kinda seems to fit, doesn’t it?