  Reflection on the role that considerations of background theory play in determining how it is reasonable for one to respond to new information have convinced some that the normative notion of evidence is better understood in terms of a three place-relation rather than a two-place relation. According to this view, judgements of the form ‘E is evidence for H’—when this is understood as more or less synonymous with ‘E tends to make it more reasonable to believe H’— are typically elliptical for judgements of the form ‘E is evidence for H relative to background theory T’. Thus, given that your background theory includes the claim that Koplik spots are a reliable indication of measles, the fact that a particular patient has Koplik spots constitutes normative evidence for you (gives you a reason to believe that) the patient has measles. On the other hand, given that my background theory does not include the claim that Koplik spots are a reliable indication of measles, the fact that the same patient has Koplik spots does not constitute normative evidence for me (give me a reason to believe that) the patient has measles.