 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 time, only one event, for any non-instantaneous event will be made of events that are its parts. Similarly for properties or tropes, there can never be only one because any one entails the existence of others, as, for example, the existence of a red property (instance) entails the existence of (an instance of) the property colour. By contrast, there could exist through a period of time just one substance, provided that it was atomic.