  This sort of abstraction Norris calls “modal abstraction.” Since we cannot imagine a body without some shape and some dimensions, shape and dimensions are determinables, or modes, of extension. Shape is not an independently existing thing, but is always a modification of an independently existing thing. Since Norris holds that our clear and distinct mental propositions track ideal propositions, we can conclude that shape and dimensions are not substances, but modes. Modal abstraction turns out to underscore Norris' rehearsal of real distinction. His account is noteworthy because it offers an alternative understanding of the real distinction proof. When we are certain that the two items under consideration are not distinct by modal abstraction, we can be sure the two items are really distinct.