The other sort of variables, called "set variables" or "class variables" or "predicates" and usually written in upper case, refer to classes/predicates/properties of individuals, which can be thought of as sets of natural numbers.
The other sort of variables, called "set variables" or "class variables" or "predicates" and usually written in upper case, refer to classes/predicates/properties of individuals. Individuals can be thought of as sets of natural numbers.