During the Song Dynasty, Zhu Xi's teachings were considered to be unorthodox. Rather than focusing on the "Book of Changes" like other Neo-Confucians, he chose to emphasize the Four Books: the "Great Learning", the "Doctrine of the Mean", the "Analects of Confucius", and the "Mencius" as the core curriculum (thing to learn) for aspiring scholar officials. For all these classics he wrote extensive commentaries (big reviews) that were not widely recognized in his time; however, they later became accepted as their standard commentaries. The Four Books served as the basis of civil service examinations all the way down to 1905.