In the late 19th century, a brilliant mathematician languished in an asylum. His greatest accomplishment, the result of a series of leaps of insight, was his pioneering understanding of the nature of infinity. This is the story of Georg Cantor: how he came to his theories and the reverberations of his work, the consequences of which shape our world. Cantor's theory of the infinite is famous for its many seeming contradictions: for example, we can prove there are as many points on a line one inch long as on a line one mile long; we can also prove that in all time there are as many years as there are days. According to Cantor, infinite sets are equal. The mind-twisting, deeply philosophical work of Cantor has its roots in ancient Greek mathematics and Jewish numerology as found in the mystical work known as the Kabbalah. Cantor used the term aleph-the first letter of the Hebrew alphabet, with all its attendant divine associations-to refer to the mysterious number which is the sum of positive integers. It is not the last positive number, because . . . there is no last. It is the ultimate number that is always being approached: just as, for example, there is no last fraction before the number 1.