In the historical development of set theory, it was mentioned that Russel demonstrated, the following classical definition is not sufficient since it leads to paradoxical constructs. If you are a beginning student of the set theory, the classical definition of Cantor is a good starting point, because it is highly intuitive.

(Original, naive set definition of Cantor (1895))^{1}

A **set** is a combination of well-distinguishable, mathematical objects. Let \(X\) be a set.

- If an object \(x\) belongs to the set \(X\), it is called ist
**element**and written as \(x\in X\). - We write \(x\notin X\), if \(x\) is not an element of the set \(X\).
- If $X$ has no elements, we call $X$
**empty**, and write $X=\emptyset,$ otherwise**non-empty**and write $X\neq\emptyset.$

^{1} Nowadays, we use the Zermelo-Fraenkel axioms (ZFA) to define sets.

| | | | | created: 2014-03-22 15:09:49 | modified: 2019-09-07 08:12:09 | by: *bookofproofs* | references: [656], [7838]

[656] **Hoffmann, Dirk W.**: “Grenzen der Mathematik – Eine Reise durch die Kerngebiete der mathematischen Logik”, Spektrum Akademischer Verlag, 2011

[7838] **Kohar, Richard**: “Basic Discrete Mathematics, Logic, Set Theory & Probability”, World Scientific, 2016