Usbek to Rhedi in Venice
The world, my dear Rhedi, is not incorruptible ; even the heavens are not. Astronomers are eyewitnesses to all the changes that are the very natural effects of the universal movement of matter.
The Earth is subject like the other planets to the same laws of motion. It suffers within itself a perpetual combat of its principles ; sea and continent seem to be eternally at war ; every instant produces new combinations.
Men in a dwelling so subject to change are in an equally uncertain state : a hundred thousand causes can act, the smallest of which can destroy them, and a fortiori increase or diminish their numbers.
I will not talk to you about those individual catastrophes, so common in the histories, that have destroyed cities and entire kingdoms ; there are general ones that have many times put the human race within an inch of its life.
The histories are full of those universal plagues that have by turns ravaged the world. They speak of one among others that was so violent it burned plants to the very roots and was felt throughout the known world, as far as the empire of Cathay ; one degree more corruption might have destroyed all of humankind in a single day.
Less than two centuries ago the most shameful of all diseases manifested itself in Europe, in Asia, and in Africa.  In a very short time it had prodigious effects ; men were finished if it had continued its advance with the same fury. Stricken with illnesses from birth, unable to sustain the weight of social functions, they would have perished miserably.
What if that poison had been a little more potent ? And it doubtless would have become so, if they had not been fortunate enough to find a remedy as powerful as the one that was discovered.  Maybe that disease attacking the reproductive organs would have attacked reproduction itself.
But why speak of the destruction that could have come to the human race ? Did it not indeed occur, and did the Flood not reduce it to a single family ?
Can those who are familiar with nature, and have a reasonable notion of God, understand that matter and things created should date back no farther than six thousand years ?  That God should have postponed his handiwork for all eternity, and only yesterday made use of his creative power ? Could it be because he could not have, or because he did not want to ? But if he was unable to at one time, he was unable at another : it is therefore because he did not want to. But as there is no succession in God, if we allow that he could have wanted something once, he has always wanted it, and from the beginning.
Therefore we must not count the years of the world ; the number of grains of sand in the sea is not more comparable to them than an instant.
Nevertheless all the historians speak to us of a first father ; they depict for us the beginnings of humankind. Is it not natural to think that Adam was saved from a common calamity as Noah was from the Flood, and that these great events have been frequent on earth since the creation of the world ?
I have had the satisfaction of offering you these general notions before I reply more specifically to your letter on the diminution of peoples that has occurred in the last seventeen or eighteen centuries ; I shall show you in a later letter that, independently of the physical causes, there are moral causes that have produced this effect.
Paris this 8th day of the moon of Chahban 1718