A Note on Irrelevance in Inductive Logic

We consider two formalizations of the notion of irrelevance as a rationality principle within the framework of (Carnapian) Inductive Logic: Johnson’s Sufficientness Principle, JSP, which is classically important because it leads to Carnap’s influential Continuum of Inductive Methods and the recently proposed Weak Irrelevance Principle, WIP. We give a complete characterization of the language invariant probability functions satisfying WIP which generalizes the Nix–Paris Continuum. We argue that the derivation of two very disparate families of inductive methods from alternative perceptions of ‘irrelevance’ is an indication that this notion is imperfectly understood at present.