Abstract

Summary: Responding to Laudan’s skeptical reading of history an influential group of realists claim that the seriously wrong claims past successful theories licensed were not really implicated in the predictions that once singled them out as successful. For example, in the case of Fresnel’s theory of light, it is said that although he appealed to the ether he didn’t actually need to in order to derive his famous experimental predictions—in them, we are assured, the ether concept was “idle,” “inessential,” “peripheral” or worse. This view, developed by J. Worrall and P. Kitcher in the 1980s and subsequently supplemented by J. Leplin and by S. Psillos has received critical attention in the literature over the last decade, but more needs to be said on the subject—or so I suggest in this paper. I bring forward four converging argumentative lines to show how and why, from the days of Fresnel to at least the decade after the Michelson-Morley experiments, the ether functioned and was understood as an “essential” posit in physics. My first line draws Fresnel’s actual deployment of the ether concept and the way he and his circle understood the achievements of his theory. The second line draws is from epistemological assessments of surprising implication in theories and its impact on leading theorists in the last two-thirds of the century. The third line draws from discussions of the optical ether in end-of-century reports circa 1900. The fourth focuses on entrenched metaphysical assumptions that persisted in the practice of physics until the advent of special relativity. Pulling these four lines together shows, I think, why attempts at synchronic identification of sound theory-parts are bound to fail, and also how realists might try to meet the challenge this creates.