When playing chess, there is a strategy for cashing out counterfactuals of the form "If I make this move", which involves considering the rules of chess, and the assumption that your opponent will make the best available move. The problem is to come up with a general method of cashing out counterfactuals that works in more general situations than playing chess. It does not work to just compute logical consequences because any conclusion can be derived from a contradiction. So a concept of counterfactuals should specify what other facts must be modified or ignored to avoid deriving a contradiction. The strategy used for chess achieves this by specifying the facts that you may consider.