I said something yesterday that isn't right, as I realized this morning. I said, ". . . a necessary condition of a term's being a sortal is that it be such that, if it applies to a thing, then it does not apply to the proper parts of the thing."
What I said works for some examples. 'Red thing,' 'physical object,' and 'entity' are not sortals. A red thing can easily have proper parts that are red things. The proper parts of a physical object are physical objects. The proper parts of entities are themselves entities. And so on.
But isn't 'rope' a sortal? If I have a ten foot rope and cut into two equal pieces, then I have two ropes. The same goes for 'rubber hose,' 'cloud,' 'amoeba.' (These latter examples from Nicholas Griffin, Relative Identity, Oxford, 1977, p. 38.) So it cannot be true that, if 'T' is a sortal, then you cannot divide T into two parts and get two Ts.
And you thought I never admitted mistakes?