 By “sum” here he almost certainly means the mereological sum, not the result of physical juxtaposition. This would mean that “object” and “space time region” would not be sortals, but “spatio-temporally continuous object” and “space-time region with volume less than x” are sortals. These results may be consistent with Quine’s views given that he has indicated that he accepts “object”, but they diverge from the Strawson-Wiggins intent. Note that the three terms listed above have some significant differences; “water” refers to a kind of stuff, “footwear” to a kind of thing, and “red” is a property of both stuff and things. (See Laycock 2011 and the entry on  object  for further discussion.)