Full Idea
Lewis thinks it is a major defect in a theory of causation that it builds in the condition that the time of the cause precede that of the effect: that cause precedes effect is something we ought to explain (which his counterfactual theory claims to do).
Gist of Idea
A theory of causation should explain why cause precedes effect, not take it for granted
Source
report of David Lewis (Causation [1973]) by Hartry Field - Causation in a Physical World
Book Reference
'The Oxford Handbook of Metaphysics', ed/tr. Loux,M /Zimmerman,D [OUP 2005], p.454
A Reaction
My immediate reaction is that the chances of explaining such a thing are probably nil, and that we might as well just accept the direction of causation as a given. Even philosophers balk at the question 'why doesn't time go backwards?'