Substitutable means to an outcome become causally relevant only when none is available.

Evidence

The second main category of two-level theories uses substitutable relationships at the secondary level. To use the multiple roads metaphor, the various roads to Rome (since “all roads lead to Rome”3) are not causes of getting to Rome, but the alternatives available to travelers for getting to Rome. These roads can play a role in causal explanations. For example, the absence of some people from Rome might be explained by the absence of any road to Rome. So while the substitutable relationship is not itself causal, it can easily play a role in causal explanation of individual cases. — Gary Goertz, Social Science Concepts and Measurement, 1570

Categorization

• SEED: Angle: road-network metaphors for equifinality — how ‘how many paths exist’ itself becomes an explanatory variable in case studies.