 There are many possible responses to this argument, but here I will briefly mention only three. The first is the ability hypothesis due to Lawrence Nemerow (1988) and developed and defended by David Lewis (1994). The ability hypothesis follows Ryle (1949) in drawing a sharp distinction between propositional knowledge or knowledge-that (such as ‘Mary knows that snow is white’) and knowledge-how (such as ‘Mary knows how to ride a bike’), and then suggests that all Mary gains is the latter. On the other hand, P2 would only be true if Mary gained propositional knowledge.