 Worrall’s mature account is based on a view of scientific theories that he credits to Duhem—which claims that a scientific theory is naturally thought of as consisting of a core claim together with some set of more specific auxiliary claims. It is commonly the case that the core theory will leave undetermined certain ‘free parameters’ and the auxiliary claims fix values for such parameters. To cite an example Worrall often uses, the wave theory of light consists of the core theory that light is a periodic disturbance transmitted though some sort of elastic medium. This core claim by itself leaves open various free parameters concerning the wavelengths of particular types of monochromatic light. Worrall proposes to understand the diminished status of evidential support associated with accommodation as follows: when evidence e is ‘used’ in the construction of a theory, it is typically used to establish the value of a free parameter in some core theory T. The fixed version will be a specific version \(T'\) of T. e serves to confirm \(T'\), then, only on the condition that there is independent support for T—thus accommodation provides only ‘conditional confirmation’. Importantly, evidence e that is used in this way will by itself typically provide no evidence for core theory T. Worrall (2002: 201) offers as an illustration the support offered to the wave theory of light (W) by the two slit experiment using light from a sodium arc—the data will consist of various alternating light and dark ‘fringes’. The fringe data can be used to compute the wavelength of sodium light—and thus used to generate a more specific version of the wave theory of light \(W'\)—one which conjoins W with a claim about the wavelength of this particular sort of light. But the data offer merely conditional support to \(W'\)—that is the data support \(W'\) only on the condition that there is independent evidence for W. 