1, 2, 3, 4, . . . , natural numbers. 0, zero. -1, -2, -3, -4, . . . , negative integers. Rational numbers, irrational numbers, real numbers, complex numbers, . . . , and, what are numbers? The most accurate mathematical answer to the question is given in this book. The book is intended to be a supplement to textbooks on the differential and integral calculus. Those who are studying the subject should read this book. However, prerequisites are kept minimum, challenging high school students are also welcome. This emended edition is newly typeset and corrected. Asymmetry of the book cover is due to a formal display problem. Actual books are printed symmetrically. Please look at the paperback edition for the correct image. The free PDF file available on the publisher's website www.bwpest2018.org