Vaught Robert L. Set Theory: an Introduction

2nd ed. — Boston: Birkhauser, 1985. — 170 p.Set theory has two overlapping aspects. In one, it is a branch of mathematics, like algebra or differentia] geometry, with its own special subject matter. In its other aspect, set theory is not a branch of mathematics but the very root of mathematics from which all branches of mathematics rise. (In this picture only logic lies still below set theory. Together they are often called the 'foundations of mathematics).