Ne was the original negative particle and is still present on its own in certain fixed expressions, like je ne sais quoi.
In more recent usage (compared to the 12th-century grammaticalization of pas, Voltaire is recent!), it doesn't strike me as random, but rather poetic.
It can still be used today with a similar effect; compare its appearance in « Beau comme le soleil », a song from the 1998 musical Notre-Dame de Paris:
Quand il me serre contre lui
je voudrais fuir, mais je ne puis.
Here the use of puis instead of peux also reflects the elevated register of a lone ne.
Another theory I might propose for that particular usage of Voltaire's is that de ta vie could function as a sort of grammaticalized negative particle. That is to say, if jamais can complete ne in place of pas, then it's conceivable that de ta vie, which has a similar meaning, could do so too. There is quite a variety of items that can fill that slot, and the Banque de dépannage linguistique article that Laure links to below shows de sa vie being used in the same way:
Il n’a de sa vie imaginé qu’il serait un jour un héros.
But I'm not quite sure I'm interpreting the Voltaire sentence correctly, since the structure could be approcher quelqu'un de X instead of de ta vie being temporal. Also, in Laure's catalogue below and in the links cited, it's not clear that this phenomenon would be at home in the imperative mood.