Duhem, Pierre. To Save the Phenomena: an essay on the idea of physical theory from Plato to Galileo (trans. Edmund Doland and Chaninah Maschler; Chicago: University of Chicago Press, 1969).
This is a classic work of philosophy of science, incorporating ideas first published by French physicist Pierre Duhem in 1908. He was widely read in the history of science, and in the content translated here studies the long legacy of a concept first emerging with Greek philosophers such as Posidonius and Simplicius, and to some extent in Plato, certainly his later advocate Proclus, but opposed by Aristotle. The idea is that the astronomer, and we could read here any physical scientist, is simply trying to offer hypotheses that explain what is observed about the natural world, in this case the movement of the planets in the heavens. The opposite is to claim that the hypothesis one offers to explain and integrate such observations corresponds to ‘the way things are’ – which is to adhere to ‘realism’. Greek philosophy was as a rule sympathetic to the goal of simply offering explanatory models that didn’t claim to be ‘real’ or explain true causes, because they mostly believed that the heavens, from the moon’s orbit and upwards, was fundamentally different to the terrestrial (earthly) sphere and so in principle could not be grasped by human thought. It was the realm of God or the gods, and they alone knew how it truly worked. Someone like Ptolemy, most importantly, was happy if he could offer a geometrical model that might explain the way the ‘planets’ (among which they counted the sun and the moon) moved. They debated whether such models should depend mostly on elliptical orbits around the earth, or instead, circular orbits around which planets moved while themselves orbiting ‘epicycles’, a compound cyclical motion that seemed truer to the fundamentally circular nature that seemed appropriate to heavenly motion. A few proposed that the earth itself might move, but that wasn’t the general mood.
Duhem shows (or claims), most interestingly, that around the time of Copernicus, many astronomers still acknowledged this principle that they were obliged simply to offer a mathematical model that explained planetary paths. Some Aristotelians in the Averroist tradition, called Peripatetics, maintained a realist view instead. Copernicus himself, claims Duhem, was a true realist, and believed that his new heliocentric theory (following the minority classical antecedents) actually represented the true realities of the cosmos. This was a little unusual at the time, and even the anonymous author of the preface to Copernicus’ groundbreaking De revolutionibus (1543), in which his new model was published, heads in exactly the opposite direction, presenting Copernicus’ theory as a mere model, intended only to ‘save the appearances’. Duhem cites another astronomer as revealing the writer’s identity as the well-known Osiander. Copernicus’ proposal was rather popular among astronomers early on, and not too controversial in that most took it as simply a model, such as Osiander. It even acted as the mathematical background for the Gregorian (i.e. papal-sponsored) reform of the calendar in the 1580s. But realism was on the rise, and Duhem uses this to help explain why Giordano Bruno and especially Galileo got in trouble with the Catholic church. By insisting that the Copernican model was not just a mathematical help for astronomy but represented cosmic realities truly, they forced it to impinge on the territory of natural philosophy, concerned with real causes. In the day, this meant it confronted both Aristotelian metaphysics, which was fundamentally geocentric, and biblical statements that seemed to speak of the earth’s fixity. And it was compatible with neither.
So Galileo probably could have co-existed with church authorities had he been a non-realist, offering simply to ‘save the phenomena’. It was his stout realism that got him into trouble! But Duhem points out that he and his colleagues, even Tycho Brahe in offering a compromise claim, effectively unified physics by seeking common explanations for the movements of heavenly and earthly things. No longer would the heavens be seen as fundamentally incomprehensible. It was game on for modern astronomy.