In the English language, the works of Shakespeare have been a particularly fertile ground for textual criticism—both because the texts, as transmitted, contain a considerable amount of variation, and because the effort and expense of producing superior editions of his works have always been widely viewed as worthwhile. The principles of textual criticism, although originally developed and refined for works of antiquity, the Bible, and Shakespeare, have been applied to many works, extending backwards from the present to the earliest known written documents, in Mesopotamia and Egypt—a period of about five millennia. However, the application of textual criticism to non-religious works does not antedate the invention of printing. While Christianity has been relatively receptive to textual criticism, application of it to the Jewish (Masoretic) Torah and the Qur'an is, to the devout, taboo.[citation needed]