The development of Islamic calligraphy is strongly tied to the Qur'an; chapters and excerpts from the Qur'an are a common and almost universal text upon which Islamic calligraphy is based. Although artistic depictions of people and animals are not explicitly forbidden by the Qur'an, pictures have traditionally been limited in Islamic books in order to avoid idolatry. In the ancient world, though, artists would often get around this prohibition by using strands of tiny writing to construct lines and images. Calligraphy was a valued art form, even as a moral good. An ancient Arabic proverb illustrates this point by emphatically stating that "Purity of writing is purity of the soul."