"On the one hand, the author of the letter is a sort of eye, a sort of source of supervision for the person to whom he addresses his advice and opinion. Seneca tells Lucilius: When I send you a letter giving you advice, in a way it is as if I myself were coming to see you and check what you are doing. But on the other hand, inasmuch as he recounts his own life, what he is doing, his choices, hesitations, and decisions, the person who writes the letter puts his own life under the watchful eye of his addressee. The two correspondents, author and addressee of the missive, are thus subject to each other’s watchful eye. Correspondence is a practice of the true life as unconcealed life, that is to say, as life under the both real and virtual eye of the other."
Michel Foucault, The Courage of Truth: Lectures at the Collège de France, 252