In the Epistle that precedes the play, Jonson dedicates Volpone to Oxford University and Cambridge University, calling them "most equal Sisters" (line 12). To begin, he states that wit alone cannot be transcendent. Rather, it requires a time, a place, and a favorable audience. Thus, he says, a poet must incline himself toward these things. Jonson thanks Cambridge and Oxford for accepting his work.
He then decries the "too much licence of poetasters in this time" (lines 23-4). By this, he means that his contemporaries do not observe the rules of dramaturgy. He says that, to be done well, this kind of uninhibited writing requires great skill and therefore should not be attempted too often. In one of the more significant lines of the Epistle, Jonson aphorizes, "For, if men will impartially, and not asquint, look toward the offices and function of a poet, they will easily conclude to themselves the impossibility of any man's being the good poet without first being a good man" (lines 29-33).
Jonson goes on to define a good poet as an inspiring, well-mannered teacher of ethics and laments that none of "the writers of these days" (line 41) fit this bill. He takes part of the responsibility for the low quality of the theater and admits it would be slander to say that all of his contemporaries are responsible. He says he will be called sharp, proud, and bitter for deriding his contemporaries, but asks his critics to consider that he has not deprived them of their dignity. He maintains his innocence.
Jonson then claims he would rather stick to his principles and be unknown than stoop to a lower quality of writing and become famous. He quotes Horace, saying "Although he is uninjured, everyone fears for himself and is angry." He emphasizes how serious he is because his reputation as well as the reputations of his contemporaries are on the line. Jonson states that the "principal end of poesie [is] to inform men in the best reason of living" (lines 111-2). He says that though the denouement of Volpone may be criticized, it is not without its purpose. He comments that it is the "office of a comic poet to imitate justice, and instruct to life, as well as purity of language, or stir up gentle affections" (lines 123-5). Lastly, Jonson passionately vows to resurrect the art of poetry.
The most important points to emerge from the Epistle are Jonson's statements regarding the duty of a poet and the purpose of comedy. As Volpone proves, Jonson believed that all drama was meant to be didactic, that is, to instruct viewers in the proper way of living. Thus, even comedies are supposed to moralize.
If Jonson seems defensive about Volpone's ending, it is because Volpone's ending is unconventional for a comedy. In general, Elizabethan comedy operated on the principle all's well that ends well. However, the ending of Volpone is certainly not happy. Jonson defends this by saying that in ancient times, comedies did not always happily. He also affirms that his unhappy ending follows from his didactic purpose: he wishes to show us the just desserts of greed and deception, and as virtually every character in Volpone is a greedy deceiver, it's only natural that none of them come to a good end.
In any case, Jonson's vow to "raise the despised head of poetry again" (line 131) is somewhat presumptuous considering that poetry, or at least stage poetry, i.e. theater, was quite popular at the beginning of the 17th century. Today's reader should consider that Jonson's definition of 'poetry' is narrower than ours. He considers true poetry to follow Classical ideals -- that is, to serve a didactic purpose and to follow the Classical unities of time, place and action. His vow to restore poetry, then, is really just a promise to revive this Classical model of good drama. His promise makes perfect sense in the context of his dedication: Cambridge and Oxford, the defenders of Classical values in Renaissance England, would be quite receptive to his stated love of the Classics, and his promise to transform the English theater into a more erudite, genteel and serious institution. (Incidentally, Jonson's plays, though popular, certainly did not alter English drama as seismically as he promises -- which is, perhaps, a good thing.)