 A system of ontological categories drawn out in any of these modes has the potential for a great many uses in philosophy, but those who would offer such systems of categories also face a variety of difficulties. They must address the issue of what the proper methods are by means of which categories are to be distinguished, how many categories there are and what they are, whether or not there is a single summum genus subsuming all other categories, and whether we should distinguish a single system of categories or multiple dimensions of categories – issues on which there has been little agreement.