Agathon and Plato's Banquet on the Prairie

In the prairie dialogue from March 1853, "lover of antithesis" has long perplexed me.  Now, aha! I find this very attribute assigned to the Greek dramatist and poet Agathon in the first volume of Bayle's Dictionary:

AGATHON, a Tragic and Comic Poet [A] Disciple of Prodicus (a), and Socrates (b), is greatly celebrated for his Beauty in Plato's Dialogue (c), in which he is farther commended for his Good Nature (d)....The Pieces of This Poet were so full of Antitheses, that he replyed one day to a Person, who was for expunging them, You are not aware, that you rob Agathon of himself (k). (Bayle)

UDAPTE 1:  Merton M. Sealts, Jr. ranks Bayle's Dictionary as "the most important secondary source of Melville's knowledge of ancient thought" ("Melville and the Philosophers" in Pursuing Melville).  In 1849, in Boston, Melville bought a set of his own. 

In Bayle the phrase "lover of antithesis" occurs when quoting from the (incorrect, according to Bayle)  Dictionary of Charles Stephens, thus:

 "Agatho, the Pythagorean Philosopber, a great Lover of Antithesis..."

Agathon is the host at Plato's Banquet (Symposium) and delivers one of the panegyrics on Love.  Famed for his own personal beauty, Agathon among other things "makes Eros beautiful and a lover of beauty."  (R. E. Allen)

From The Theatre of the Greeks  (Cambridge, 1830):

With Euripides, as far as we are concerned, the History of ancient Tragedy comes to an end, though there were many more recent Tragedians, for instance, Agathon, whom Aristophanes describes to us as perfumed all over, and crowned with flowers, and into whose mouth Plato, in his Symposion, puts a speech in the taste of the sophist Gorgias, full of the most exquisite elegancies and unmeaning antitheses. He was the first that forsook Mythology, as the natural material of the drama, and sometimes wrote tragedies with purely fictitious names, (this is to be noticed as forming a transition to the newer Comedy) one of which was called the Flower, and probably, therefore, was neither seriously touching, nor terrible, but of an idyllic and pleasing character.

UPDATE 2: Again as shown by Sealts, the remark about hugging "ugly Socrates" in Melville's November 1851 letter to Hawthorne makes for "an unmistakable allusion to the Symposium, or Banquet," supplying "clear evidence that Melville was reading Plato in Pittsfield."  ("Melville and the Platonic Tradition")  More recently, Beverly R. Voloshin finely considers the importance of Plato's Banquet for Melville in her 2011 Leviathan article, "Parables of Creation:  Hawthorne, Melville, and Plato's Banquet."

Back on the prairie,
The Captain's expressed pity for one who escapes suffering ("pitiable exempt from love's misery") makes for an antithesis or paradox worthy of Agathon.  The Captain's antithetical style reminds Frank of Agathon; his themes of beauty and love allusively recall the philosophical talk at Plato's Banquet.

F.—"Now, listen to the song of that bird; it will soothe your nerves."

C.—"Nerves! It is medicine to the mind!—it comes like a message of love!"

F.—"Nay, there, we have agreed to disagree."

C.—"Thou pitiable exempt from love's misery, thou believest in beauty?"

F.—"Yes, thou unintelligible lover of antithesis, (not to say plagiarism.)"

C.—"Is any thing so beautiful as unbounded faith?"

F.—"Listen! that's 'to horse.'"

C.—"Answer me then!"

F.—"Pshaw !—Of course it's beautiful; or rather, sublime."

C.—"It is the very attribute of human love!"

July 8th.—Those who lack faith that the above was dreamed, spoken and scribbled, as described, lack, too, experience of the human mind, and prairie or desert influences and feelings.

"Scenes Beyond the Western Border," March 1853 and (revised)
Scenes and Adventures in the Army