Full Idea
Lewis agrees that properties cannot be sets of their actual instances, but claims they can be sets of their possible instances. This would distinguish coextensive properties like being cordate and renate, since they might be separated.
Clarification
Renate means 'has kidneys' and cordate means 'has a heart'. In the actual world they are never found separately
Gist of Idea
Properties are sets of their possible instances (which separates 'renate' from 'cordate')
Source
report of David Lewis (Modal Realism at Work: Properties [1986]) by DH Mellor / A Oliver - Introduction to 'Properties' §10
Book Reference
'Properties', ed/tr. Mellor,D.H. /Oliver,A [OUP 1997], p.23
A Reaction
Sounds wrong. Two properties could be obviously different even if they could never be separated. In this world a creature might briefly survive without kidneys.