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)
You may also be interested in Edmund Burke's essay A Philosophical Enquiry into the Origin of Our Ideas of the Sublime and Beautiful (1757). In it, he differentiates the beautiful from the sublime. The beautiful, he argues, gives pleasure more or less by being smooth and nicely formed. It offers an aesthetic experience that is pleasing but not challenging. The sublime, in contrast, is something that inspires both awe and terror--a massive thunderstorm, for example, or the view into the heart of a mountain. Most simply, the sublime arises from the experience of encountering something can destroy us.
Burke's ideas of the sublime greatly influenced the artists and writers of the Romantic period, who turned away from the Neoclassical emphasis on beautiful form and line to expressive emotion. Broadly, it's the difference between, say, Mozart (the beautiful) and later Beethoven (the sublime).
Longinus regarded the sublime as a thing of spirit, a spark leaping from writer to reader, rather than a product of technique. He lists five sources of the sublime, the first two of which--great thoughts and noble feelings--are gifts of nature and the last three of which --lofty fiures, speech, diction and arrangement--are products of art.
Longinus identified five sources of the sublime:
"the power of forming great conceptions";
"vehement and inspired passion";
"the due formation of figures";
"noble diction"; and
"dignified and elevated composition."
Correspondingly, he highlights three problem areas to avoid while seeking the sublime:
Tumidity (overblown language)
Puerility (hair splitting) and Parenthyrsus (false passion)
In essence, he realizes the same great men who produce great ideas will, by nature of their 'greatness' also be capable of deep and sincere feeling.
To achieve this end, Longinus believed in expressing thoughts and feelings via noble language ("due formation of figures"), all of which is designed to elevate the individual into the transdecent: "a figure is at its best when the very fact that it is a figure escapes attention."
To achieve this "formation of figures" there are six figure types:
inversions of word order
polyptota--accumulations, variations, and climaxes
particulars combined from the plural to the singular
interchange of persons--addressing the audience as "you"
periphrasis (circumlocution)--wordiness, circling around the issue
Hope that helps.