The Seljooqids established the first and widest Iranian Empire after Islam and the Strongest power of the eastern territory of the Islam world. During the middle ages, the seljooqid government was successful in political and military fields. After overcoming the Ghaznavis, they even conquered the Abbassides, occupied Baghdad and made it their own territory. By following the Abbassids' governors superficially, they reinforced their authority and political legitimacy. They fought their civil foes, specially the Esmaillids and foreign powerful rivals like Egyptian Fatemids. The crusades occurred during their dynasty. Hence, their governing method, particularly feudal system, led to their weakness, deterioration, and finally their fall. One of the most important issues of the seljooqid dynasty was the depression of development of rational sciences, particularly philosophy, dialectics and intellectuality. The basis of this depression and tendency began in the Ghaznavid era. It has lots of reasons including the political aims of the seljooqids, their religious bias, religious and governmental policies of the Abbassid governors, from Motevakkel and his followers, the opposition of titleholders such as khwaja Nezamolmolk with rational sciences the suppression of Mo'atazela, the domination of Ashari word, the enhancement of social classification and being superficially religious, the abusing of religion in order to suppress rivals, the suppression of the Shiites, the opposition of the scholars (clergymen), the jurists, the literary men and poets with philosophy and ralional sciences.