Meanwhile, back at the ranch...
Pierre Duhem's To Save The Phenomena acquaints the reader with primary sources in the history of science - Aristotle, Plato, Augustine, Averroes, Thomas Aquinas, Copernicus. It really is a treat to read quotes from these folks. One surprise to me is just how sophisticated the discussions were back 600 or a thousand years ago. These discussions took a long time to work through human consciousness before any fundamental change in human thinking was accomplished.
The fundamental change that Duhem focused on is the change from believing that any theory about the motion of the planets and stars was merely a geometric fiction meant to describe the paths of those objects in the sky, to believing that that geometric "fiction" was instead a statement about the nature of those objects.
The former belief regarding theories of the motion of planets and stars was like a tennis instructor saying, "When you serve, move your right arm as if you are catching a butterfly up in a tree, and bringing your butterfly net down to the ground." The point of that "visualization" is to get your arm to follow a very precise path each time you serve.
In the same way, when folks who paid close attention to how planets travel through the field of stars over days and months and years, made up a system of patterns that, if consistently applied to those planets, would predict the actual path of those planets over time. For those ancient astronomers, they said, "Imagine the planets being carried around in concentric spheres of differing sizes, and those spheres each have their own, systematic, consistent motion. When all those spheres are moving, in their separate motions, they carry the planet inside in a certain path." The system of spheres were called eccentrics and epicycles.
The descriptions and calculations of those eccentrics and epicycles were geometric, and follow accepted geometric laws and axioms. The calculations were quite sophisticated, and eventually, quite accurate sytems of prediction - when would Venus rise and set, during what time of year, etc..
The thing about all this was that there was more than one system of eccentrics and epicycles. But each system could fairly accurately predict where the planets would be. Though even from the time of the ancient Greeks there was argument as to whether one system might be true and the others false, the argument was not a serious ontological issue in and of itself. That is, for the most part, most astronomers accepted that these systems were as convenient, and as "real", as the tennis instructor's "butterfly net".
The fundamental change that Duhem focused on is the change from believing that any theory about the motion of the planets and stars was merely a geometric fiction meant to describe the paths of those objects in the sky, to believing that that geometric "fiction" was instead a statement about the nature of those objects.
The former belief regarding theories of the motion of planets and stars was like a tennis instructor saying, "When you serve, move your right arm as if you are catching a butterfly up in a tree, and bringing your butterfly net down to the ground." The point of that "visualization" is to get your arm to follow a very precise path each time you serve.
In the same way, when folks who paid close attention to how planets travel through the field of stars over days and months and years, made up a system of patterns that, if consistently applied to those planets, would predict the actual path of those planets over time. For those ancient astronomers, they said, "Imagine the planets being carried around in concentric spheres of differing sizes, and those spheres each have their own, systematic, consistent motion. When all those spheres are moving, in their separate motions, they carry the planet inside in a certain path." The system of spheres were called eccentrics and epicycles.
The descriptions and calculations of those eccentrics and epicycles were geometric, and follow accepted geometric laws and axioms. The calculations were quite sophisticated, and eventually, quite accurate sytems of prediction - when would Venus rise and set, during what time of year, etc..
The thing about all this was that there was more than one system of eccentrics and epicycles. But each system could fairly accurately predict where the planets would be. Though even from the time of the ancient Greeks there was argument as to whether one system might be true and the others false, the argument was not a serious ontological issue in and of itself. That is, for the most part, most astronomers accepted that these systems were as convenient, and as "real", as the tennis instructor's "butterfly net".
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