As a Biographical Document, it is worth a very strict perusal, if you are interested that way in either Friedrich or Voltaire: finely significant hints and traits, though often almost evanescent, so slight are they, abound in this Correspondence; frankness, veracity under graceful forms, being the rule of it, strange to say! As an illustration of Two memorable Characters, and of their Century; showing on what terms the sage Plato of the Eighteenth Century and his Tyrant Dionysius correspond, and what their manners are to one another, it may long have a kind of interest to mankind: otherwise it has not much left.
In Friedrich's History it was, no doubt, an important fact, that there lived a Voltaire along with him, twenty years his senior.

With another Theory of the Universe than the Voltaire one, how much OTHER had Friedrich too been! But the Theory called by Voltaire's name was not properly of Voltaire's creating, but only of his uttering and publishing; it lay ready for everybody's finding, and could not well have been altogether missed by such a one as Friedrich.

So that perhaps we exaggerate the effects of Voltaire on him, though undoubtedly they were considerable.

Considerable; but not derived from this express correspondence, which seldom turns on didactic points at all; derived rather from Voltaire's Printed WORKS, where they lay derivable to all the world.

Certain enough it is, Voltaire was at this time, and continued all his days, Friedrich's chief Thinker in the world; unofficially, the chief Preacher, Prophet and Priest of this Working King;--no better off for a spiritual Trismegistus was poor Friedrich in the world! On the practical side, Friedrich soon outgrew him,--perhaps had already outgrown, having far more veracity of character, and an intellect far better built in the silent parts of it, and trained too by hard experiences to know shadow from substance;--outgrew him, and gradually learned to look down upon him, occasionally with much contempt, in regard to the practical.