There are variations between religious (schools) and marked differences between different branches of Islam. Aniconism is common among fundamentalist Sunni sects such as Salafis and Wahhabis (which are also often iconoclastic), and less prevalent among liberal movements within Islam. Shia and mystical orders also have less stringent views on aniconism. On the individual level, whether or not specific Muslims believe in aniconism may depend on how much credence is given to hadith, and how liberal or strict they are in personal practice. Aniconism in Islam not only deals with the material image, but touches upon mental representations as well. It is a problematic issue, discussed by early theologians, as to how to describe God, Muhammad and other prophets, and, indeed, if it is permissible at all to do so. God is usually represented by immaterial attributes, such as "holy" or "merciful", commonly known from his "Ninety-nine beautiful names". Muhammad's physical appearance, however, is amply described, particularly in the traditions on his life and deeds recorded in the biographies known as . Of no less interest is the validity of sightings of holy personages made during dreams. Titus Burckhardt sums up the role of aniconism in Islamic aesthetics as follows: In practice Religious core In practice, the core of normative religion in Islam is consistently aniconic. Spaces such as the mosque and objects like the Quran are devoid of figurative images. Other spheres of religion, for example mysticism, popular piety, or private devotion exhibit significant variability in this regard. Aniconism in secular contexts is even more variable and there are many examples of figural representation in secular art throughout history. Generally speaking, aniconism in Islamic societies is restricted in modern times to specific religious contexts. In the past, it was enforced only in some times and places. Past The representation of living beings in Islamic art is not just a modern phenomenon and examples are found from the earliest periods of Islamic history. Frescos and reliefs of humans and animals adorned palaces of the Umayyad era, as on the famous Mshatta Facade now in Berlin. The ‘Abbasid Palaces at Samarra also contained figurative imagery. Ceramics, metalware, and objects in ivory, rock crystal, and other media also bore figural imagery in the medieval era. Figurative miniatures in books occur later in most Islamic countries but somewhat less in Arabic-speaking areas. The human figure is central to the Persian miniature and other traditions