Ben Jonson's comedic Jacobean play Volpone (an Italian word meaning "The Big Fox") is introduced with an Epistle (epistle means letter, especially a formal one) that sets the philosophic background for Jonson's choices of structure for and principles in Volpone. Jonson thought an epistolary explanation was necessary for several reasons. These reasons were that he diverged from the then current comedic formulation; he disapproved of the laxity evident in his contemporaries poetry; he desired adherence to a more ancient purpose of play writing.
In comedic plays contemporaneous with Ben Jonson, the endings were resolved happily; sad endings were reserved for tragedies. Jonson correctly asserted that ancient Greek comedies sometimes had unhappy endings if an unhappy ending harmonized with the moral being instructed in the play. Jonson advocated adherence to the ancient types and to Aristotle's unities of time, place and action in which the play should cover no more than a twenty-four hour period of time. Jonson's ending, as he explains in the Epistle, breaks with his contemporaries and has a Greek-style unhappy ending. The Epistle serves as an apologia of this choice. Jonson asserts his choice is the only possible ending that could demonstrate his moral that teaches against greed and deception.
The second concern that Jonson addresses in his Epistle is the laxity of poetic works written by his contemporaries. In the previous Elizabethan age, Edmund Spenser, Phillip Sidney and William Shakespeare all adhered to the mimetic theory described by Sidney in his essay "An Apology for Poetry." In essence, poets are gifted with divine inspiration and in their poetry imitate (mimesis) the eternal truths toward which all human hearts and souls aspire, thereby acting as liaisons between truth and goodness and humanity. Jonson adhered to this mimetic theory while his contemporaries were finding it less compelling and writing verse that didn't live up to the Elizabethan standard. It is important to note that while plays were then written in poetry, not all poetry was in a play; this acknowledgment avoids confusion over the word "poetry" referring only to plays.
Jonson's displeasure, as stated in the Epistle, with his contemporaries plays was also founded on the disagreement about whether, in light of the mimetic quality of poetry, all plays (comedic or dramatic) must teach an uplifting moral. His contemporaries were writing comedies with happy endings and no instructive moral. Jonson criticized this failure and advocated that even comedies should present moral lessons, as Volpone does.