Longinus' Peri Hupsous, or "On the Sublime" is a Greek treatise, probably written sometime between the first and third centuries AD. The sole surviving manuscript attributes it to one Dionysius or Longinus; some scholars identify the author as the third century philologist Cassius Longinus, but the attribution is still disputed.
The treatise is strikingly original. It begins by claiming (correctly) that previous writers on the art of rhetoric have neglected to discuss sublimity. It then argues that sublimity is the single most important element in writing, and goes into some detail as to where sublimity may be found (quotations of passages the author considers sublime) and how it is acheived.
The emphasis on sublimity fits well with the neoplationic preoccupation, seen in Plotinus` `On Beauty`and Proclus`Commentary on Plato`s Republic with understanding the artistic as well as philosophical traditions of Hellenic paideia as modes by which the soul could reach the divine; consequently, they develop elaborate theories of types of audience and art which imitate the forms directly, rather than imitate the sensibilia, and thus form a mode by which the receptive and philosopphically trained audience can attain knowledge of the forms.
This shift in Platonizing philosophy for opposed to art to seeing art as a form of theological knowledge was extremely influential, shaping Byzantine iconodule theory, and much of early Renaissance literary criticism (Petrarch, Dante, Ficino, Sidney, etc)