Monday, May 11, 2009

Duhem on Ptolemy

I need to take a break from grading, so what better than to talk about books and philosophy of science?

I recently finished To Save the Phenomena, an old book by Pierre Duhem which I had wanted to read for some time now. Duhem offers what appears (deceptively, it seems—see below) to be a detailed account of the history of astronomy from the ancient Greeks all the way to Galileo, and argues that there are two conflicting approaches to astronomy which persist throughout: one that takes good theories to be literally true, and one that takes good theories merely to be adequate to account for everything we see (that is, to "save the phenomena"), without necessarily being literally true. So, for instance, when one considers the Ptolemaic system, with all of its epicycles, one might argue that the epicycles represent real, physical things (like crystalline spheres), or one might argue that they are just mathematical devices to help us calculate the complex actual paths of the planets. Likewise, when one looks at the Copernican system, one might take it to say that the Earth actually does move, or, again, one might contend that the motion of the Earth is just an abstract assumption that makes the calculations easier. If you have some background in philosophy of science, you will recognize the two approaches as scientific realism and instrumentalism, respectively.

The book excited me from the first chapter, because Duhem claimed that Ptolemy himself was an instrumentalist, which is the opposite of what I had always heard. Alas, further investigation revealed that this claim (and more) had been refuted by G.E.R. Lloyd ("Saving the Appearances," The Classical Quarterly, New Series, Vol. 28, No. 1 (1978), pp. 202-222), and that Duhem apparently had already distanced himself from it in his later Aim and Structure of Physical Theory. Duhem's book has also been criticized for the chapters on the Arabs and on Copernicus, though I have not read through those particular critiques, yet. It's an interesting project, trying to figure out who exactly was a realist and who was an instrumentalist, and a good idea to have secure answers before passing on the examples to one's students.

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