Sentence:  But <WSD>Aristotle</WSD> kept the principle of levels and even augmented it by describing in the Poetics what kinds of character and action must be imitated if the play is to be a vehicle of serious and important human truths . 
Meaning of the word marked with <WSD> tags:  one of the greatest of the ancient Athenian philosophers; pupil of Plato; teacher of Alexander the Great (384-322 BC)