 At any rate, Hume never explored the alternative counterfactual approach to causation. In this, as in much else, he was followed by generations of empiricist philosophers. The chief obstacle in empiricists’ minds to explaining causation in terms of counterfactuals was the obscurity of counterfactuals themselves, owing chiefly to their reference to unactualised possibilities. The true potential of the counterfactual approach to causation did not become clear until counterfactuals became better understood through the development of possible world semantics in the early 1970s.