 Confucianism and Buddhism were, however, always closely interrelated throughout Japanese history. The introduction of Confucianism, for example, came alongside of the introduction of Buddhism. Later the Neo-Confucian teachings of various Song philosophers including the Cheng brothers, Zhu Xi, and Lu Xiangshan, were introduced to Japan beginning in the thirteenth century, by Zen monks who had traveled to China for advanced study of Chan teachings there. For centuries, the teachings of the Song philosophers remained subjects of study largely within the confines of Zen monasteries, attaining no independent existence or philosophical integrity apart from Zen. This should hardly be surprising, given that Neo-Confucian philosophers such as Zhu Xi had themselves come to develop their Confucian thinking after earlier study and practice of Chan Buddhism. While Zhu Xi came to criticize Buddhism, other Neo-Confucians did not see Buddhism as necessarily antithetical to Confucianism or Neo-Confucianism. Ultimately, however, many Neo-Confucians did come to be more distinctive and unrelenting critics of Buddhism than tolerant competitors.