Usagi Keikaronjo - A book on the theory of cormorant poetry

…Both of these became key words in medieval poetry theory as critical terms expressing aesthetic ideals, but their meanings were not necessarily clear, and they also grew in ways that deviated from the aesthetic ideals that Shunzei and Teika intended to convey with the words. They appear in spurious works that bear the name of Teika, such as Guhisho and Sangoki, which are known as the Usagi-style poetry theory books. However, compared to the critical terms expressing aesthetic ideals in the Heian period, such as 'Aware', 'Taketakasi', and 'Okashi', 'Yugen' and 'Ushin' have a marked tendency toward internalization and a strong ideological quality, which makes them distinctive as medieval concepts. Taking this into consideration, 'Yugen' and 'Ushin' discussed in the Usagi-style poetry theory books also had important significance in tracing the general outline of the history of poetry theory. …

*Some of the terminology used in the "Usagi-kei Poetry Theory Book" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…いずれも美的理念を表す批評語として，中世歌論のキー･ワードとなってゆくが，その意味する内容はかならずしも明確ではなく，また，俊成，定家がその語に託そうとした美的理念からずれたかたちでそれが成長を遂げてゆきもした。《愚秘抄》《三五記（さんごき）》等，いわゆる鵜鷺（うさぎ）系歌論書と呼ばれる定家の名に仮託した偽書にあらわれるそれである。ただ，平安朝時代の美的理念をあらわす批評語であった〈あはれ〉〈たけ高し〉〈をかし〉等と比べると，〈幽玄〉〈有心〉はいちじるしく内面化の傾向を強め，思想性を濃厚にもつ点で中世的理念としての特色をもつのであって，その点を考慮に入れれば，鵜鷺系歌論書に論じられた〈幽玄〉〈有心〉もまた，歌論史の大筋をたどる上で重要な意義をもったのであった。…

※「鵜鷺系歌論書」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報