 To appreciate Kenny’s conclusion, it is instructive to work through why precisely this is a counterexample to (MA). Consider an agent S who has the ability to pick a red card or a black card, but does not have the ability to pick a red card or the ability to pick a black card. According to (MA), S has the ability to A iff S does A at some world (or set of worlds) satisfying condition C. Consider the case where (MA) appeals to a single world, not a set of worlds. If S has the ability to pick a red card or pick a black card, then by (MA) there is a world w satisfying condition C where S picks a red card or black card. Then either S picks a red card at w or S picks a black card at w. Then, applying (MA) now in the other direction, S has the ability to pick a red card or S has the ability to pick a black card.  But that, by assumption, is not the case. Since (MA) is the only substantive premise appealed to that argument, (MA) must be rejected.