Full Idea
If Kennedy had still been President in Dec 1963, he would not have been assassinated in Nov 1963, so the counterfactual goes both ways (where the cause seems to only go one way).
Gist of Idea
If the effect hadn't occurred the cause wouldn't have happened, so counterfactuals are two-way
Source
Tim Maudlin (The Metaphysics within Physics [2007], 5)
Book Reference
Maudlin,Tim: 'The Metaphysics within Physics' [OUP 2007], p.144
A Reaction
Maudlin says a lot of fine-tuning has sort of addressed these problems, but that counterfactual causation is basically wrong-headed anyway, and I incline to agree, though one must understand what the theory is (and is not) trying to do.