"Truth brings joyous tranquility"

Iblis asked, "Can you tell a lie from the truth,
you who are filled with illusion?"
Mu`awiyah answered, "The Prophet has given a clue,
a touchstone to know
the base coin from the true.
He has said, "That which is false troubles the heart,
but Truth brings joyous tranquility."
- Rumi

گفت چون دانی دروغ و راست را
ای خیال اندیش پر اندیشه ها
گفت پیغمبر نشانی داده است
قلب و نیکو را محک بنهاده است
گفته است الکذب ریب فی القلوب
گفت الصدق طمأنین طروب

The question of what is truth and what is lie, what is right and what is wrong, what is bad and what is good, all are in the same group I guess. Although Rumi (he is better known as Mawlana in Iran) says that we have a simple clue or touchstone (Benchmark?) to figure out what is what, I dare say it is nothing easy at all and it is everything that can make one spiritually happy and prosperous or miserable and lost.

There is a funny thing in the translation, it says Mu`awiyah is answering to Iblis (devil), but in the original Persian verses there is no reference to Mu`awiyah. May be this is a follow up of a different part from which the translator deduced that devil is talking to Mu`awiyah. (Mu'awiyah was the founder of the Umayya dynasty of caliphs. He is famous for his clashes and war with Imam Ali and he and his son are the symbol of ultimate evil in Shiite Islam.)