Mohammed the Conqueror expressed the same idea poetically in a letter to Uzun Hassan: “Our empire is the home of Islam; from father to son the lamp of our empire is kept burning with oil from the hearts of the infidels.”[162] This declaration seems to reveal two things. First, the Conqueror asserts that, since by Moslem theory there can be but one _Dar ul-Islam_, or land of Islam, his empire is the sole lawful Moslem state; second, he declares that, by the policy of his house, the empire derives its strength from the ever-renewed supply of Christians. Whether this exegesis be exact or not, the fact is indisputable that the fundamental missionary spirit of Islam was strong in the Ottomans of the sixteenth century,[163] and that the Ruling Institution was deliberately conducted for the purpose, among others, of transferring the ablest and most useful of the subject Christians in each generation into the dominant nation. As the first Western observer who comprehended the system remarked, “This comes from no accident, but from a certain essential interior foundation and cause, which,” he feels it his Christian duty to say, with a helpless admiration, “is desperation of good, and obstinacy in evil, and ... is the work of the devil.”[164] Not only did Mohammedanism encourage the practice of taking in outsiders to serve, fight, and aid in ruling, but this practice was thoroughly in harmony with the old Turkish spirit which prevailed in the steppe lands, and a similar policy had been followed by the Byzantine Empire. Thus, in encouraging the incorporation of foreigners the three great influences which met in the Ottoman state had exerted a combined activity as perhaps in no other direction. The Ruling Institution acted for centuries as a great steadily-working machine for conversion. OTHER MOTIVES FOR INCORPORATING CHRISTIANS