 The basic issue of the book is a philosophical analysis of the concept of number, which is the most basic concept on which the entire edifice of arithmetic and mathematics can be founded.In order to proceed with this analysis, Husserl, following Brentano and Stumpf, uses the tools of psychology to look for the "origin and content" of the concept of number.He begins with the classical definition, already given by Euclid, Thomas Hobbes and Gottfried Wilhelm Leibniz, that "number is a multiplicity of unities" and then asks himself