In mathematics, the category Rel has the class of sets as objects and relations as morphisms. Euclid, Greek mathematician, 3rd century BC, known today as the father of geometry; shown here in a detail of The School of Athens by Raphael. ... Look up category in Wiktionary, the free dictionary. ... In mathematics, a set can be thought of as any collection of distinct things considered as a whole. ... Category theory is a mathematical theory that deals in an abstract way with mathematical structures and relationships between them. ... In mathematics, a finitary relation is defined by one of the formal definitions given below. ...