Thinking of not having a false heart means the supported reason is true. (In the songs of five commandments, Fumogokai (a Buddhist admonition not to tell a lie), "Shingosen Wakashu" Volume 9, Shakyoka (waka about Buddhism)).
いつはりの　心あらじと思ふこそ　たもてる法のまことなりけれ　（五戒歌の中に、不妄語戒　『新後撰和歌集』　巻第九　釈教）