Is knowledge the ability to φ for the reason that p?

Hyman (1999, 2006) argues that knowledge is best conceived as a kind of ability: S knows that p iff S can φ for the reason that p. Hyman motivates this thesis by appealing to Gettier cases. I argue that it is counterexampled by a certain kind of Gettier case where the fact that p is a cause of the subject's belief that p. One can φ for the reason that p even if one does not know that p. So knowledge is not best conceived as an ability of this kind.