A Euclidean comparison theory for the size of sets

Abstract

We discuss two main ways in comparing and evaluating the size of sets: the "Cantorian" way, grounded on the so called Hume principle (two sets have equal size if they are equipotent), and the "Euclidean" way, maintaining Euclid's principle "the whole is greater than the part". The former being deeply investigated since the very birth of set theory, we concentrate here on the "Euclidean" notion of size (numerosity), that maintains the Cantorain defiitions of order, addition and multiplication, while preserving the natural idea that a set is (strictly) larger than its proper subsets. These numerosities satisfy the five Euclid's common notions, and constitute a semiring of nonstandarda natural numbers, thus enjoying the best arithmetic. Most relevant is the natural set theoretic definition} of the set-preordering: $$X\prec Y\ \ \Iff\ \ \exists Z\ X\simeq Z\subset Y$$ Extending this ``proper subset property" from countable to uncountable sets has been the main open question in this area from the beginning of the century.