[W]hat makes the relationship between Saichō and Kūkai decisive in Japanese Buddhist history is not so much their cooperation as the manner in which it came to an end. Their alliance began to deteriorate when Saichō, after receiving abhiseka from Kūkai, hurried back to Mount Hiei, where the work of laying the foundation of the new Tendai school awaited him. Saichō continued to study and copy Mikkyō texts borrowed from Kūkai, but despite Kūkai's repeated requests he did not return to Takaosan-ji to resume his studies. Their rapport finally terminated when Kūkai harshly condemned Saichō's approach to Mikkyō as a transgression of the esoteric precept of samaya [the promise to keep the oral/esoteric teachings private], and Saichō retorted by denouncing Kūkai's manner of instruction