  The most direct challenge to moral knowledge is to question whether anything exists that could possibly serve as a suitable object of such knowledge. If no conception of what moral reality would have to be like fits our understanding of the kinds of things that exist, then moral knowledge is not possible. In general, the candidates for possible objects of moral knowledge divide into three categories: natural (objects that are knowable only through experience), non-natural (but not supernatural), and theological (or supernatural). We will assess reasons for thinking that none of these categories provide suitable objects of moral knowledge, treating them in reverse order.