The most important book in the philosophy of science since World War II is without a doubt Thomas Kuhn’s The Structure of Scientific Revolutions. In his book Kuhn argues that when a scientific revolution occurs it is a complete paradigm shift. Even though old ideas may still appear applicable, any scientific revolution is fundamentally incommensurable with earlier scientific thoughts. This means that any new idea forms a completely new paradigm, or way of thinking. Terms may stay the same, but a new scientific paradigm is profoundly and fundamentally distinct from what came before. Although historians and philosophers have debated Kuhn’s ideas since his book’s publication, studying various cases of scientific revolutions quickly show that Kuhn’s incommensurability model is the most accurate in describing how science changes. One of the best examples is the Copernican Revolution.
Before Copernicus the accepted model of the universe was the old Ptolemaic model. The Earth was the center of the universe. All mobile heavenly bodies circled it. Copernicus turned the universe inside out, placing the sun at the center and the Earth in orbit around it, sharing orbital characteristics with the other known planets. Does Copernicus represent a classic Kuhnian revolution? To answer the question we need to look at the characteristics of the Copernican model in comparison the Ptolemaic model. Obviously these two models are different, but to prove Kuhn right, historians must show that these two models are completely incommensurable. The biggest incommensurability between these two models is the order the universe. Clearly one cannot be a Ptolemaic and a Copernican at the same time. The position of the sun and Earth in both models is completely different and incommensurable. There is no way for the two philosophies to interact.
But when it comes down to the underlying ideas of the two models, they seem to be talking about the same thing. Both Ptolemy and Copernicus recognized that objects orbit around each other. Both scientists believed (incorrectly) that the orbits were circular. Both models have a similar conception of the nature of stars with minor differences. Besides the placement of the heavenly objects, everything else seems exactly the same.
At first glance this might seem true, but digging deeper shows that even though the concepts are the same, the two models are discussing different things. Take orbits. In the Ptolemaic model an orbit was something that objects did around the Earth, with no real explanation. Copernicus did not have the right explanation for orbits, but by placing the sun at the center of the solar system Copernicus allowed future models to develop. Newton and Kepler would have never been able to do their work in an Earth centered universe. Newton’s law of gravity and Kepler’s orbital equations do not work if the larger Sun orbits around the smaller Earth. The ability of the Copernican model to allow for future developments make it incommensurable with the Ptolemaic model. Although Copernicus did not know it, his correct conception of orbits started modern astronomy. Only in his model do gravitationally determined orbits work. A Copernican orbit inherently includes the idea that larger objects attract smaller ones, even though Copernicus did not know that. Therefore even though a Copernican orbit and a Ptolemaic orbit use the same word “orbit”, the structure of that entity is completely different.
A critique of Kuhn could easily point out that even though Copernicus represented a fundamental philosophical change he did not represent a fundamental practical change. Copernicus’s model was no better at prediction than the Ptolemaic model and was in fact more complex. If both models are able to find the same results, can we really say that they are incommensurable?
Even though the predictions in Copernicus’s time came out nearly the same as the Ptolemaic predictions, this could not have lasted forever. On a very superficial level the two theories were commensurable, but Kuhn is not talking about superficial differences between scientific theories. He is talking about the deep fundamental assumptions that theories make about the nature of reality. Ptolemaic predictions may have been correct in Copernicus’s time, but modern astronomy could not have developed within that system. It needed to take the next step into the Copernican system. The two systems appeared similar, but at a deep level they were completely different. As mentioned above, the vocabulary was describing different things and the predictive power of the early system failed when it reached a certain level. Any overlap in predictive characteristics between the two models is just a coincidence. At the fundamental level they described a different universe. Once the Copernican model took hold, it allowed scientists like Kepler to do “normal science” within the paradigm.
The Copernican model is a perfect example of a Kuhnian revolution. Like any scientific idea, Kuhn’s model is not perfect, but it is the best explanation to how scientific revolutions occur. Although Copernicus did not make a huge predictive change, the world view of the human race changed completely.