It is amusing to note that my first encounter with On the Sublime occurred while reading the Panegyricus, and not in class. Barely a few paragraphs into the Isocratic treatise (broken up into small sections on the Perseus database), a very large and prominent footnote appeared. It read:
“The author of the treatise On the Sublime quotes this passage and condemns Isocrates’ ‘puerility’ in thus dwelling on the power of rhetoric and so arousing distrust of his sincerity…Plutarch attributes to Isocrates the definition of rhetoric as the means of making “small things great and great things small” … a similar view is attributed to Tisias and Gorgias.”
Immediately apparent from this passage was Longinus’ intent, personal convictions, and faction association in the philosophy vs. rhetoric divide. Despite all of Isocrates’ insistence to the contrary, Longinus depicted him as a sophist in the style of Gorgias, devoid of all claim to the notion of sublimity. On the Sublime itself certainly did not disappoint in these regards. Longinus specifically singled out one of Isocrates’ passages as an example of a rhetorical figure executed badly, while Plato was referenced with high praise many times in the treatise.
Furthermore, the positions advocated by Longinus were almost always diametrically opposed to those advocated by Isocrates (and by extension Cicero and Quintilian). The most prominent of these appeared to be the role of natural ability in the development of the sublime, the model citizen, or the ideal orator. Out of five sources of sublimity, Longinus attributed the foremost to “natural greatness”, while Isocrates was adamant in the Antidosis that “credit is won not by gifts of fortune but by efforts of study”.
Other criticisms of the Isocratic style I will accept and take to heart, but I cannot let this foolhardy sentiment of Longinus to stand. In fact, I consider this line of thinking to be seriously detrimental to education itself. In America, the belief that you are predisposed to failure or success is especially prevalent. Students abandon all hope on a subject too often, simply because they have been ingrained with the notion that you must be born with the “gift” in order to succeed. While natural greatness may be a non-negligible factor in reaching virtuosity or sublimity, the vastly dominant contribution comes from the sheer amount of time spent honing a skill, drilling a technique, or practicing a routine.
Talent may get you past the basic motions effortlessly, if only for the sake of appearance and bluster, but conceptual mastery is the direct result of intensive study.
There is yet another matter regarding Longinus and Plato that I must address. My primary critique of Plato was the following: while Plato had explicated in great detail his Theory of Forms, he ultimately left no way for humans to distinguish what constituted a Form and what did not. We could perhaps speculate that that the concept of sublimity is capable of being extended to enhance Plato’s Forms.
Longinus makes several references to the true sublime, which can remain awe-inspiring to its audience no matter how many times it is examined. The true sublime is independent of who or what perceives it; everyone who experiences it can only depart with unanimous and fervent praise. Can this be used a criterion in recognizing the Forms? How good of you to patch up the numerous holes in Plato’s so-called theories, Longinus!
And what a criterion it is. Will there ever be a speech, performance, philosophy, or ideal that elicits total approval from its evaluators? Why, perhaps we should journey around the globe to put certain promising items to the test, accosting all beings that chance upon us. When we’ve reached a thousand bodies in favor and only one opposed, can we call it a Form? No? Longinus, you have simply redrawn the asymptote and redefined the indefinite. What we need is a measure of how close we are to that asymptote, and correspondingly, when to stop wondering if what we have is a Form or not. I would like to have awarded you the honor of improving upon Plato, just as Cicero improved on Isocrates, Quintilian on Cicero, and Augustine on them all – but it is not to be.
Linda's Isocratean Reflections 
Two thoughts: (1) You’re right, not only about the connections b/t Longinus and Plato (and the shared problem), but about the epistemological aporia they face — they can only say “you’ll know it when you see it.” (2) This said, your statement — “What we need is a measure of how close we are to that asymptote, and correspondingly, when to stop wondering if what we have is a Form or not” — seems to accept the premises of Platonic epistemology, while criticizing its conclusions. Again, given the first post, I am surprised by the way in which you keep on pulling us back to Plato (through Isocrates); but this surprise, again, is not a criticism, and indeed I perhaps should not be surprised given the historical connections between the two figures, especially via Socrates.