:This article gives an introduction to what mathematicians call "intuitive" or "naive" set theory; for a more detailed account see Naive set theory. For a rigorous modern axiomatic treatment of sets, see Axiomatic set theory. A set is a collection of distinct objects considered as a whole. Sets are one of the most fundamental concepts in mathematics. The study of the structure of sets, set theory, is rich and ongoing. Having only been invented at the end of the 19th century, set theory is now a… (More on Set)