 Another challenge is the direction of causation. David Lewis (1973, 1979) thought that counterfactuals themselves yield an account of the direction of causation. This account, however, turned out to be rather controversial and is rarely adopted (see, e.g., Frisch 2014: 204n). To determine the direction of causation, regularity and inferential theories tend to rely on broadly Humean constraints on the temporal order between cause and effect. This move comes at the cost of excluding backwards-in-time causation. We must wonder how severe this problem is. So far, there is neither a commonly agreed understanding of the very idea of backwards causation nor actual empirical evidence for it (cf. the entry on  backwards causation).  The direction of causation remains a point of controversy within regularity, inferential, and counterfactual approaches.