What Counterfactuals Can Be Tested

TL;DR

This paper characterizes which counterfactual statements can be empirically tested by deriving their probabilities from physical experiments, addressing conceptual and practical challenges.

Contribution

It offers a complete framework for identifying testable counterfactuals and provides procedures to express their probabilities using experimental data.

Findings

Complete characterization of testable counterfactuals

Procedures to infer probabilities from experiments

Clarification of conceptual difficulties in testing counterfactuals

Abstract

Counterfactual statements, e.g., "my headache would be gone had I taken an aspirin" are central to scientific discourse, and are formally interpreted as statements derived from "alternative worlds". However, since they invoke hypothetical states of affairs, often incompatible with what is actually known or observed, testing counterfactuals is fraught with conceptual and practical difficulties. In this paper, we provide a complete characterization of "testable counterfactuals," namely, counterfactual statements whose probabilities can be inferred from physical experiments. We provide complete procedures for discerning whether a given counterfactual is testable and, if so, expressing its probability in terms of experimental data.