Oyama classifies the historical sources about Shotoku Taishi into two groups of 'Jushichijo Kenpo' written in the "Nihonshoki" and 'the halo inscription of the statue of Yakushi Zo in Horyu-ji Temple, the halo inscription of the statue of Shaka Sanson Zo in Horyu-ji Temple, Tenjukoku Shucho (embroidery representing Tenjukoku paradise), and Sangyo Gisho' in Horyu-ji Temple, and argues that both groups were created in the ages quite after the days of Umayatoo.
また、聖徳太子についての史料を『日本書紀』の「十七条憲法」と法隆寺の「法隆寺薬師像光背銘文、法隆寺釈迦三尊像光背銘文、天寿国繍帳、三経義疏」の二系統に分類し、すべて厩戸皇子よりかなり後の時代に作成されたとする。