Also, while earlier writers on the subject, had by and large confined themselves to their own school of thought, only referring intermittently, if at all to other schools, Viswanatha, in "Sahityadarpana", explicitly discussed all schools and thoughts of Indian aesthetics, before arguing the superiority of the "dhvani" school. There too, Viswanatha, does not agree with the equal importance given to the three types of "dhvani" as classified by ninth century Kashmiri aesthetician, Anandavardhana, who in his book, "Dhvanyālóka", actually established the "dhvani" school of poetics. Viswanatha concludes that "rasa dhvani" is what defines poetry.