In Book Two, Section 70 of the Masnavi, Rumi explores the distinction between truth and falsehood through Mu'awiya's dialogue with Iblis. Citing a prophetic hadith ('Falsehood is a doubt in the hearts, while truth is a tranquil joy'), Rumi explains that the human heart naturally finds peace in truth and becomes restless or disturbed by lies. A spiritually healthy heart acts as a touchstone (محک), intuitively distinguishing the 'taste' of truth from the deception of falsehood. If the heart is spiritually sick, however, it loses this discerning palate.