The purpose of this paper is to show how text contrasts and anomalies led Al Jáhiz to draw rules for translators and to examine the circumstances which contributed to the conception of his theory, on the basis of two major guidelines, similarity and contrast. Al Jáhiz was an illustrious Arab writer and chief of a religious sect, who had an encyclopaedic knowledge. He was shocked by the patterns textuality followed. He wrote his book Al-bayán wa al-tabyin (on rhetoric), after criticizing translations of a book on animals which was partly based on translations. In his book, he summarized the foundations of adequate translation in an equation with two elements: al-bayán "intelligent speech" in translation and the knowledge of the subject. One of his main objectives was to prevent translators from corrupting the language and to ensure proper textuality. He drew attention to the difficulties in translating the Quran and the intricacies of translating poetry.