Counterfactual theories define causation in terms of a counterfactual relation. These theories can often be seeing as "floating" their account of causality on top of an account of the logic of counterfactual conditionals. This approach can be traced back to David Hume's definition of the causal relation as that "where, if the first object had not been, the second never had existed." More full-fledged analysis of causation in terms of counterfactual conditionals only came in the 20th century after development of the possible world semantics for the evaluation of counterfactual conditionals. In his 1973 paper "Causation," David Lewis proposed the following definition of the notion of "causal dependence":