Counterfactuals and Unphysical Ceteris Paribus: an Explanatory Fallacy

Abstract

I reconsider a type of counterfactual argument often used in historical sciences on a recent widely discussed example of the so-called “rare Earth” hypothesis in planetary sciences and astrobiology. The argument is based on the alleged “rarity” of some crucial ingredient for the planetary habitability, which is, in Earth’s case, provided by contingent evolutionary development. For instance, the claim that a contingent fact of history which has created planet Jupiter enables shielding of Earth from most dangerous impact catastrophes, thus increasing Earth’s habitability, leads often to the conclusion that such state-of-affairs must be rare in the Galaxy. I argue that this reasoning is deeply flawed, for several closely related reasons. In addition, the relevance of the philosophical problem of transworld identity for this kind of historical reasoning in science is put forward. This highlights many explanatory problems one faces when using historical counterfactuals in study of complex, nonlinear dynamical systems – and bolsters the relevance of philosophy for evaluation of scientific explanatory claims.