They criticized Jindai-moji (written language claimed to have existed before the introduction of Chinese characters) used in the so-called Koshi Koden (unofficial ancient documents) including Takeuchi-monjo (alleged record of the lineage of ancient gods and a dynasty preceding Emperor Jinmu) for 'having only five vowels and not adopting the ancient Kana usage of eight vowels' and denied it as having been forged in a later era when such distinction of Kana usage had disappeared.
いわゆる古史古伝の竹内文書などに使用されている神代文字も「上代には8母音あったはずなのに、なぜか5母音のままで上代の仮名遣いに配慮していない」ということから、そうした仮名遣いの区別がなくなった後世の偽書として否定した。