YAKIRI's theory is based on an article within the book entitled, "Posthumous Writings of Ieyasu" (from March 25, 1614), where it is written that "Hidetada's eldest son was Takechiyo, born from Kasaga no Tsubone, and was the 3rd Shogun Iemitsu, Minister of the Left."
そのうちの「神君家康公御遺文」（慶長十九年二月二十五日付）に、「秀忠公御嫡男　竹千代君　御腹　春日局　三世将軍家光公也、左大臣」と記されていることを根拠としている。