Whether these objections to the above versions of the independence account can be answered is unclear. Hoffman and Rosenkrantz (1994 and 1997) develop a different, though probably not incompatible, conception of the relative independence of substances compared to other categories. Substance, they say, is the only category of thing that might have only one instance through at least a minimally extended period of time. Thus, for example, there is no possible world that has, in a given period of ti