Nothing  found more favor in Persian Letters than to discover in them, unexpectedly, a sort of novel. One sees the beginning, the development, and the end ; the various characters are placed in a chain that connects them. As their stay in Europe lengthens, the ways of this part of the world assume in their minds a less wondrous and less bizarre appearance, and they are more or less struck by that bizarreness and that wonder in function of the difference of their characters. Elsewhere, disorder mounts in the seraglio in Asia in proportion to the length of Usbek’s absence, in other words as the ire increases and love diminishes.
Moreover, these sorts of novels succeed ordinarily because one gives his own account of his present situation, which makes the reader feel the passions better than all the relations that one could make about them.  And that is one of the reasons for the success of a number of charming works which have appeared since the Persian Letters. 
Finally, in ordinary novels, digressions can be permitted only when they themselves constitute a new novel. Reflections cannot be worked in, because none of the characters having been assembled for reflection, it would clash with the design and the nature of the work. But in the form of letters, where the actors are not chosen, and the subjects treated are dependent on no set design or preëstablished plan, the author has given himself the advantage of being able to incorporate philosophy, politics and morality into a novel, and to connect it all by a silent and, in a sense, unknown chain.
Persian Letters had at once such prodigious sales that booksellers tried everything to procure sequels  : they would go grab everyone they met by the sleeve : Monsieur, they would say, write me some Persian letters.
But what I have just said suffices to show that they do not lend themselves to any sequel, even less to any admixture with letters written by another hand, however ingenious they may be. 
There are some barbs that numerous people have found too audacious.  But they are invited to take into account the nature of this work. The Persians who were to play such a great role in it were suddenly transplanted to Europe, which is to say into another universe. There was a time when they had necessarily to be represented as filled with ignorance and prejudice. The only concern was to make visible the generation and progression of their thoughts. Their first notions had to be singular : it seemed the only thing to do was to lend to them the sort of singularity that was compatible with cleverness. It was enough to depict the impression they had of each thing that appeared extraordinary to them. Far from thinking of implicating any principle of our religion, there was not even any inkling of imprudence. These barbs are always linked to the feeling of surprise and astonishment, and not at all with the idea of scrutiny, and even less with that of criticism. When speaking of our religion, these Persians could not appear better informed than when they spoke of our customs and manners. And if they sometimes found our doctrines singular, that singularity always testifies to their utter ignorance of the connections that exist between those doctrines and our other truths.
We make this justification out of love for those great truths, independently of the respect for the human race which it was certainly not our intention to strike in its most sensitive spot. We therefore ask the reader not to cease for a minute to look on the barbs of which I speak as effects of the surprise of men who had to feel some, or as paradoxes formed by men who were not even capable of making them. He is asked to recognize that all the pleasure lay in the everlasting contrast between real things and the singular, naive, or bizarre manner in which they are perceived. Certainly the nature and design of Persian Letters are so patent that they will never fool any who do not wish to fool themselves.