 Glymour (1999) objects that such a factory is nowhere to be found. This raises a question about the status of the Common Cause Principle. If the principle is intended to be a conceptual truth about the relationship between causation and probability, then we could undermine the principle by showing that a causal structure that violates it can be clearly conceived. On the other hand, if the principle is intended to be an empirical generalization about the relationship between causation and probability in the actual world, then Glymour is right to demand more than a hypothetical example.