When he finally responded to these allegations, it was in the form of a preface to his fifth book of madrigals, published in 1605. In this he famously laid out the distinction between the two prevalent styles, prima prattica and seconda prattica. While prima prattica dictated holding the traditional rules of music, such as part-writing, above all else, seconda prattica made it permissible to break these rules, as long as it was at the service of the text and the emotion trying to be conveyed. This dichotomy, essentially between having music serve purely an aesthetic purpose versus as a way to convey and reinforce specific and complicated emotions has been in existence ever since, and continues to be one of the dominant struggles even to this day. Even though he was no theorist himself, Monteverdi had succinctly and convincingly laid out an argument in favour of seconda prattica which would have extremely large implications later.
Although he was widely respected throughout Italy, it was not until Monteverdi completed his first opera Orfeo (1607) that he began to be recognized throughout Europe as a true genius. Although there was a small tradition of opera before the turn of the 17th century, mostly through Florentine composers such as Jacopo Peri and Giulio Caccini, there were none that could be considered masterpieces. Orfeo, which was based on the legend of Orpheus and Eurydice and featured a libretto by Alessandro Striggio, brought a whole new level of stylistic unity and an awareness of drama to the genre. Each act represented a coherent whole rather than a collection of smaller pieces, and true to his principle of seconda prattica, Monteverdi was never hesitant to employ dissonance and various instrumental and vocal techniques to add to the drama. Now over 400 years since it was first performed, Orfeo is the oldest opera to still be considered part of the standard repertoire.