Theory determines which negative cases qualify as relevant controls.

Evidence

The United States in 1900 is certainly a case of non–social revolution; should it be included in a test of her theory? Chapter 7 introduces the Possibility Principle as a solution to this problem of dividing the negative cases from the irrelevant ones. In short, one uses the theory of the positive cases to determine which of the negative ones are those where the outcome “was possible.” — Gary Goertz, Social Science Concepts, 171

Categorization

• DEFINE: The Possibility Principle is defined here: negative cases enter a test only where the theory implies the outcome ‘was possible.’
• SEED: An essay on how to pick control cases without cherry-picking: why ‘the outcome was possible’ beats ‘the outcome did not happen.’