 Lewis’s explanation of the temporal asymmetry of counterfactual dependence comes from a combination of his analysis of the similarity relation together with the (alleged) ‘asymmetry of overdetermination’ – a contingent feature of the world. According to this analysis, there are several respects of similarity to be taken into account in evaluating non-backtracking counterfactuals: similarity with respect to laws of nature and also similarity with respect to particular matters of fact. Worlds are more similar to the actual world the fewer miracles or violations of the actual laws of nature they contain. Again, worlds are more similar to the actual world the greater the spatio-temporal region of perfect match of particular fact they have with the actual world. If the laws of the actual world are deterministic, these rules will clash in assessing which counterfactual worlds are more similar to the actual world. For a world that makes a counterfactual antecedent true must differ from the actual world either in allowing some violation of the actual laws (a ‘divergence miracle’), or in differing from the actual world in particular fact. Lewis’s analysis allows a tradeoff between these competing respects of similarity in such cases. It implies that worlds with an extensive region of perfect match of particular fact can be considered very similar to the actual world provided that the match in particular facts with the actual world is achieved at the cost of a small, local miracle, but not at the cost of a big, diverse miracle.