 The Normative Constraints Question: Let us assume that there is no real threat of circularity for Williamson’s account. Several authors point out that the background knowledge that one needs to keep fixed in our counterfactual evaluations is still problematic in a number of ways. One classical question concerning the evaluation of counterfactuals is the problem of cotenability. Which facts or beliefs are cotenable with the antecedent of the counterfactual conditional? Which background knowledge or information do we hold fixed? For example, to investigate the counterfactual “If the bush had not been there, the rock would have bounced into the lake” we imaginatively consider what would happen to the rock if the bush had not been there. To do this, we imagine a scenario without the bush. That is, we dispense with the fact that the bush blocks the rock. However, do we also dispense with the fact that something (else) blocks the rock? We might hypothesize that some other obstacle gets in the way. If we retain this fact as we develop the scenario, then the rock would have stopped anyway. Yet the truth in the story that Williamson is assuming is that the rock would have bounced into the lake. So we must not hold fixed the fact about an obstacle. Why not, though? What principle determines that in Williamson’s example it is a mistake to hold fixed the obstacle fact but correct to hold fixed facts like the momentum of the rock and the presence of the lake?