From the early to the mid-Kamakura period, many karon books discussing the true essence of waka were written, among which were FUJIWARA no Toshinari and FUJIWARA no Sadaie's "Yugen" and "Ushin," which later became the foundational texts for kagaku in later generations.
鎌倉時代前期か中期にかけて和歌の本質を論じた歌論書が数多く作られたが、中でも藤原俊成・藤原定家の幽玄・有心の歌論は後世の歌学の基礎となった。