Rhetoric is the criterion or the standard of judgment for the evaluation of discourse and the beauty of language. Arabs in the pre-Islamic era were aware of and talked about literary criticism, but they have not left any written document of their criticism. After the rise of Islam and the revelation of the Quran, which is a divine miracle and unique as an example of eloquence and clarity, Iranians became the pioneers in rhetoric and many other branches of language. Metonymy is one of the most extensive arguments in rhetoric. Metonymy or the knowledge of the beauty of language is significant in Farsi, Arabic and many Western languages: To Europeans, metonymy has a broad meaning including all figures of speech, and linguists have talked about it very extensively. Metonymy, in its Islamic and Iranian sense, is of two kinds: rational and literal. Literal metonymy has its own subdivisions. Examples of metonymy from the Persian poetry up to the fifth century are given to inform the reader that in spite of the greatness of the Quran and the influence of Arabic rhetoric on Farsi, Iranian poets had their own aesthetic invention, eleganance of speech, critical mind and sound taste suggested by old idioms such as 'Farsi is Sugar".