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