What's wrong with classes? The theory of Knowledge

This paper wants to investigate the deepest meaning of the word class that is often used in machine learning and classification as a well-defined concept. This adventure will lead the reader to the fundamentals of Mathematics like set theory from Zermelo-Fraenkel. This will be our start, like is all Mathematics, to understand how well defined is the class concept. A broader theory will be outlined with the courageous attempt to give an homogenous framework to deal with machine learning problems