Sets and Classes

Abstract

Throughout this book, whenever the word set is used, it will be interpreted to mean a subset of a given set, which, unless it is assigned a different symbol in a special context, will be denoted by X. The elements of X will be called points; the set X will be referred to as the space, or the whole or entire space, under consideration. The purpose of this introductory chapter is to define the basic concepts of the theory of sets, and to state the principal results which will be used constantly in what follows.