Friedhelm Hardy in his "Viraha-bhakti" analyses the history of Krishnaism, specifically all pre-11th-century sources starting with the stories of Krishna and the gopi, and Mayon mysticism of the Vaishnava Tamil saints, Sangam Tamil literature and Alvars' Krishna-centered devotion in the rasa of the emotional union and the dating and history of the Bhagavata Purana.