The book deals with Gerbert of Aurillac (Pope Sylvester II) and his investigations in the field of theoretical (philosophy of numbers) and practical (counting) arithmetic. The book represents the comprehensive inquiry of both these aspects of arithmetic in his thought. The analysed sources are Gerbert’s so-called scientific letters written to his friends, colleagues or pupils, and also including some of his other texts. On this basis, attention is paid to arithmetic as the mother of all sciences and as the path to wisdom (e.g., the so-called Saltus Gerberti, relation between arithmetic and other disciplines of the quadrivium, etc.) and also to practical arithmetic (e.g., the introduction of Hindu-Arabic numerals and the re-introduction of a new form of abacus to the Latin Christian West).