Described as one of the great literature of humanity, including Goethe's assessment of it as one of the four main bodies of world literature, Persian literature has its roots in surviving works of Middle Persian and Old Persian, the latter of which dates back as far as 522 BCE, the date of the earliest surviving Achaemenid inscription, the Behistun Inscription. The bulk of surviving Persian literature, however, comes from the times following the Muslim conquest of Persia c. 650 CE. After the Abbasids came to power (750 CE), the Iranians became the scribes and bureaucrats of the Islamic Caliphate and, increasingly, also its writers and poets. The New Persian language literature arose and flourished in Khorasan and Transoxiana because of political reasons, early Iranian dynasties of post-Islamic Iran such as the Tahirids and Samanids being based in Khorasan.