Accommodation, Prediction and Bayesian Confirmation Theory

Abstract

‘A hypothesis can only be received upon the ground of its having been verified by successful prediction’, C.S. Peirce, Collected Papers, Vol. II, p.466.Hempel, in his classic [1966], raised a problem which already had a long pedigree. Balmer, having examined measurements of the wavelengths of lines in the emission spectrum of hydrogen, constructed a now well-known formula for generating these, and also other lines at that time unknown to him. Hempel asked whether Balmer's formula would have been as strongly confirmed by the new lines had they too formed part of Balmer's database. Hempel's question poses the famous and ancient accommodation-versus-prediction problem: is a hypothesis h, designed among other things to account for data e, confirmed by e as much as h would have been, had it been found to explain e without, however, having been constructed to do so? An unconditionally negative answer has been given by a numerous and influential body of scientists and philosophers; this unconditional view of the matter I shall call predictionism.