Arithmetic deals with numbers: but what is the nature of their existence, of their parts, and of their relationship with countable items? These questions nourished a lively debate between the Platonico-Pythagorean tradition (trying to answer them) and the Pyrrhonian tradition (trying to show that these answers were unsatisfactory). The debate lies at the heart of Sextus Empiricus' Against the Arithmeticians. The present book aims at facing the remarkable historical and philosophical questions raised by Sextus' treatise by offering a new translation of it and the first dedicated commentary to it.