 From a purely technical point of view, one can introduce different kinds of variables to reflect any distinction one wants, e.g., there could be distinctive styles of variables for even numbers as opposed to odd ones, or for left-handed people as opposed to right-handed. However, Carnap and others who advocated a many-sorted logic for reasons that were more ontological than pragmatic introduced different variables only for what they believed were metaphysically distinct kinds of entities, e.g., numbers, physical objects, sense data. In this respect, there is some parallel with ultimate sortals as discussed above, although Carnap does not make an explicit connection with earlier writers.