Not a valid argument against a hypothetical.

It's not a valid argument against its truth, but it's a valid argument against its relevance. A hypothetical is useless if its antecedent never obtains.

Smoking lesion problem?

Like I said, it's an empirical question. For philosophers, that's usually the end of the inquiry, though it's very nice when someone goes out and does some experiments to figure out which way causality goes.