In his book Kitab al-Şaydana, Abu Rayḥān Bῑrūnῑ, when recalling the weakness of the Persian language in comparison to the Arabic language, states that the Persian language is only good for telling nightly tales and narrating stories for kings, and not for presenting precise scientific and philosophical discussions and issues. Although Abu Rayhan stated this at a time when the Dari Persian language was still in the early stages of its life, and if he were to judge the Persian language two or three hundred years later, he would not have issued such a ruling, nevertheless, what he said is not without truth. In fact, if we want to express Abu Rayhan’s statement fairly, we should say that the Persian language throughout its life has made more use of literary techniques and poetic imaginations and has demonstrated its skill in the field of literature, and even when it has wanted to express philosophical concepts, it has still used allegorical stories and metaphors and what mystics and Sufis have called the language of allusion, and this has been a characteristic of the Persian language and the mentality of those who used it. This judgment that we mentioned as a characteristic of the Persian language is true both in ancient times, in the Avestan period, and in Middle Persian during the Sasanian period. One of the most vibrant and dynamic allusive expressions that has been common among Iranian philosophers and mystics both in Middle Persian and in the modern era is the expression “sea.” This name, which was used in ordinary Persian language in a known sense, has been used by Iranian scholars and mystics to refer to heavenly things and divine attributes, as well as their opposites, i.e., demonic attributes. Of course, in the pre-Islamic period, Iranian sages used the sea to name the universals that exist in the heavenly realm, which Plato and the Platonists call Forms, and in the Islamic period, our mystics mainly considered it to refer to divine attributes or states and stages of the soul, and moral perfections and vices. In this article, which is divided into two parts, we will first try to examine the meaning that was intended for “sea” in the Middle Persian language, especially in the sixth book of the Denkard, and then we will examine the function of this word in the works and sayings of Persian-speaking mystics, from Bāyazῑd Basṭāmῑ and Abu al-Ḥasan Kharqānῑ to writers and poets such as Rashid al-Din Maybodῑ and Farid al-Dῑn Aṭṭār.