 Baldwin avoids this issue with a different definition. Concrete entities are violators of Leibniz’s principle of the identity of indiscernibles. Concrete things may have exact duplicates. For instance, Cameron Winklevoss could have a twin who perfectly resembles him (unlike his homozygotic twin Tyler Winklevoss). In contrast, the unit set comprised of Cameron Winklevoss cannot have a perfect twin. All sets obey Leibniz’s principle of the identity of indiscernibles and so Baldwin counts sets as abstract entities.