A Significance-Based Account of Ceteris paribus Counterfactuals

When evaluating a counterfactual statement, it is often convenient to specify conditions that ought to be kept unchanged. Formally, this can be done by associating to each counterfactual a ceteris paribus set of formulas, specifying the facts that “ought to be kept unchanged”. Ceteris paribus counterfactuals originate in the debate between D. Lewis and Fine in the 1970s, and have been captured in formal accounts. However, these accounts are merely based on ‘counting’ formulas, and can yield counterintuitive results. In this paper, we develop a novel approach to evaluate ceteris paribus counterfactuals at (weakly) centered sphere models, by taking into account the ‘significance’ of formulas that ought to be kept unchanged. Hypothetical states that keep the most significant formulas unchanged will be prioritized in the evaluation of a counterfactual. We show that the resulting notion of validity coincides with theoremhood in Lewis’ conditional logics VC or VW.