|Set Theory: set theory is the system of rules and axioms, which regulates the formation of sets. The elements are exclusively numbers. Sets contain individual objects, that is, numbers as elements. Furthermore, sets contain sub-sets, that is, again sets of elements. The set of all sub-sets of a set is called the power set. Each set contains the empty set as a subset, but not as an element. The size of sets is called the cardinality. Sets containing the same elements are identical. See also comprehension, comprehension axiom, selection axiom, infinity axiom, couple set axiom, extensionality principle.|
Books on Amazon
Nouns/Prior: no names, no class name - Epsilon/Principia Mathematica/Russell: "x ε a": Translation: "A is an element of the class of humans" seems to be a relation between a concrete and an abstract object - Vs: better "x is a": "Russell is a man" - Prior: "is a" is no real verb that makes a sentence of a name, rather a sentence of name and noun - "ε" is not a real predicate.
Objects of thought Oxford 1971
Arthur N. Prior
Papers on Time and Tense 2nd Edition Oxford 2003