The phrase the law of the Medes and Persians denotes something which cannot be altered.

It is a reference to the Book of Daniel, Chapter 6:
Darius the Mede, King of Babylon, has elevated Daniel to high office, exciting the jealousy of the leading men of the kingdom. Knowing of Daniel’s devotion to his God, his enemies trick Darius into issuing a decree ordering that anyone who, during the next thirty days, prays to any god or human being except to the King shall be thrown into the lions’ den. As Daniel continues to pray to his God, Darius, forced by his own decree, throws Daniel into the lions’ den.

The unalterableness of the law of the Medes and Persians is mentioned as follows in the Book of Daniel, Chapter 6—the quotations are alternately from the Early Version (around 1382) of the Wycliffe Bible and from the King James Version (1611):

– Verse 8:

And so now, kyng, conferme thou the sentence, and write the decree, that it be not chaungid whiche is ordeynyd of Medis and Persis, nether be it leeful to eny man for to breke.

Now, O king, establish the decree, and signe the writing, that it be not changed, according to the law of the Medes & Persians, which altereth not.

– Verse 12:

And thei cummynge to spaken to the kyng on the maundement, Kyng, wher thou ordeynidist not, that eche man whiche preyde eny of goddis and men, vnto thritti days, no bot thee, kyng, he shulde be sent in to the lake of lyouns? To whom the kyng answerynge saith, The word is trewe, vp the decree of Medis and Persis, whiche it is not leeful for to breeke.

Then they came neere, and spake before the king concerning the kings decree; Hast thou not signed a decree, that euery man that shall aske a petition of any God or man, within thirty dayes, saue of thee, O king, shal be cast into the denne of Lions? The king answered and said, The thing is true, according to the law of the Medes and Persians, which altereth not.

– Verse 15:

Forsothe tho men vndirstondynge the kyng, saiden to hym, Wite thou, kyng, for the lawe of Medis and Persis is, that eche decree whiche the kyng ordeyneth, be not leeful for to be chaungid.

Then these men assembled vnto the king, and said vnto the king, Know, O king, that the law of the Medes and Persians is, that no decree nor statute which the king establisheth, may bee changed.

The earliest known allusive use of the law of the Medes and Persians is from The Isle of Man: or, The Legall Proceeding in Man-shire against Sinne (London, 1627), by the English Puritan pastor Richard Bernard (1568-1641):

Old-man, the Law of the King allowes thee not the benefit of the Clergie, for The reward of Sin is death: This is his Maiesties Decree, vnchangeable, as the Law of the Medes and Persians.

The image appeared in the following paragraph from The Caledonian Mercury (Edinburgh, Scotland) of Monday 15^th October 1764:

Paris, Sept. 28. Although the edicts and arrets of the French Monarch are as strict as the acts of commerce and navigation of other States, as to prohibiting foreigners from trading in their islands and colonies in America, nevertheless they do not think their laws ought to remain unalterable, according to the maxim of the Medes and Persians; for it having been represented that the Dutch, from their late prudent encouragement of foreigners in the West-Indies, have engrossed a certain very lucrative trade with the Spaniards, and even English; discretionary powers have lately been transmitted to the several officers in that part of the world, to dispense with all such laws as shall appear for the benefit of the colonies, and without prejudice to the trade and navigation of France.

—Cf. also the humorous phrase one man’s Mede is another man’s Persian.