The double aim of the paper is first to understand how is elaborated the aition notion in Aristotle’s An. Po II, 11, that is to say, not everywhere in his work but in a chapter of the Organon about the demonstration where it takes a technical meaning; second to show how its semantic framework conforms and echoes with what shapes the concept before and aside in all Ancient Greece through various historical figures. An important Greek conception, with not only epistemological but also religious, social and political meaning, underlies the notion. When a superior entity, like logos, Dikê, phusis, Gods or Destiny, shares reality, the aitios/aition is the actor or active element which, as taking a part, disturbs or changes the order. In demonstration, logos shares Being in subjects and predicates while the aition, middle term which takes a share in the major and the minor premises, provides a semantic event bringing something new in what was firstly established.