 A second response appeals to the distinction between a priori and a posteriori physicalism. As we saw above, the crucial claim of a posteriori physicalism is that (13) — i.e. the claim that S entails S* — is a posteriori. Since (13) is a posteriori, you would need certain experience to know it. But, it is argued, Mary has not had (and cannot have) the relevant experience. Hence she does not know (13). On the other hand, the mere fact that Mary has not had (and cannot have) the experience to know (13) does not remove the possibility that (13) is true. Hence a posteriori physicalism can avoid the knowledge argument. (It is an interesting question which premise of the knowledge argument is being attacked by this response. The answer depends on whether (13) is physical or not: if (13) is physical, then the response attacks P1. But if (13) is not physical, the response is that the argument is invalid.).