"Western humour theory begins with Plato, who attributed to Socrates (as a semi-historical dialogue character) in the Philebus the view that the essence of the ridiculous is an ignorance in the weak, who are thus unable to retaliate when ridiculed. Later, in Greek philosophy, Aristotle, in the Poetics, suggested that an ugliness that does not disgust is fundamental to humour."
Source: Wikipedia
Note: Humour = spelling in English from UK, while humor is the American spelling form