 The main difficulties for the diagonal proposition account concern (1) how to apply it in order to determine the narrow content of a given mental state, and (2) the fact that it gives a way of defining the truth conditions of a mental state only in centered worlds in which the state actually exists. In the example above, for instance, Oscar is presumed to be in the same mental state in each context, although differences in how he came to be in that state affect its content. But this raises the problem of what we need to “hold constant” in considering various counterfactual contexts in order to be considering a context in which Oscar is in the relevant state. These issues are considered briefly in section 5.1.