The 'Letter to Caesar' is the key to the mystery of the prophecies. It is also a codified message addressed to an individual of the future which the seer calls Caesar, the name of his elder son in that life as Nostradamus. It is not blindly that he chose that name, but rather to establish the link between his son of the 16th century and that personage of the future of which he so astutely veils the coordinates and the name.
Those who do not believe in the theory of successive lives (reincarnation) will find here another reason to denigrate the prophecies, but then, I will send them back to Copernicus and his theory that the earth and the planets revolved around the sun, when most of his contemporaries believed, with the help of the Church of course, that the earth was the center of the universe. Here, we are talking about a cosmic and multidimensional universe of birth and rebirth, but the problem will always be one of openmindedness and unbiased judgment.
When he writes,
car la parole héréditaire de l'occulte sera dans mon estomac intercluse
[The hereditary gift of prophecy will come from within my body]
Nostradamus is, in fact, saying that, by means of the genetic code?, one of his offspring will inherit the gift of prophecy, and, as we will see, this is confirmed at the end of the letter. The rest of that letter is addressed to that offspring to whom he tries to explain that the gift of prophecy is a two-edged sword, and it is not always wise to reveal the future:
si je venais a reserer ce qu'à l'avenir sera, ceux de règne, secte, religion et foi, trouveraient si mal accordant à leur fantaisie auriculaire qu'ils viendraient à damner ce que, par les siècles à venir, on connaîtra être vu et aperçu.
[If I were to reveal the future, those in authority and heads of churches and sects would find it so at variance with their auricular fancies,-- as in the expression 'my little finger tells me'-- that they would curse those events that will later on become the past and present of future centuries.]
According to Nostradamus, one can predict the future and fix events in time and space by means of astrology and astronomy:
Mais, moyennant quelque indivisible éternité, par comitiale agitation Hiraclienne, les causes par le céleste mouvement sont connues,
[but, because the eternity is indivisible, and according to Heraclitus, the cosmos being in constant change and movement, the cause (of things) is known by the movement of the stars.]
but he insists that the gift of prophecy is of divine inspiration:
...mais la parfaite des causes notice ne se peut acquérir sans celle divine inspiration...
[but the exact knowledge of causes is not possible without this divine inspiration...]
this meaning that, although astrology and astronomy were useful tools, the visions themselves were received through divine inspiration.
As to the interpreter of the prophecies himself, Nostradamus gives us two important clues. The first one has been intentionally placed at the beginning of the letter:
Et depuis qu'il a plu au Dieu immortel que tu ne sois venu en naturelle lumière dans cette terrene plage, et ne veux dire tes ans qui ne sont encore accompagnés...
[and since it is the will of God that you will be reborn on a foreign continent, and I cannot reveal when that time will be..]
Here, Nostradamus clearly states that, even though the letter is addressed to his son, Caesar, it is, actually, to the reincarnation of that son in a foreign country that he addresses these words. It is also to that individual that the seer bequeaths his gift of prophecy when he ends the 'Letter' with another prophecy:
Faisant fin, mon fils, prends donc ce don de ton père, Nostradamus, espérant toi déclarer une chacune prophétie des quatrains ici mis. Priant au Dieu immortel, qu'il te veuille prêter vie longue, en bonne et prospère félicité. De Salon, ce 1 de Mars, 1555.
[Lastly, my son, take this gift from your father, Nostradamus, hoping you will interpret each of the quatrains of this book. I pray the immortal God to grant you a long, prosperous and happy life. From Salon, this 1st of March, 1555.]
In this last excerpt, Nostradamus seems to predict that this son of the future will have the gift of prophecy. He also encourages him to interpret the prophecies.