In Technologies of the Self Foucault states:
“[In ancient Greece] it was generally acknowledged that it was good to be reflective, at least briefly…Writing was also important in the culture of taking care of oneself. One of the main features of taking care involved taking notes on oneself to be reread, …and keeping notebooks in order to reactivate for oneself the truths one needed. Socrates’ letters are an example of this self-exercise” (p. 27).
Foucault continues:
“Taking care of oneself became linked to constant writing activity. The self is something to write about, a theme or object (subject) of writing activity…this is not a modern trait born of the Reformation…it is one of the most ancient Western traditions” (p. 27).
And then:
“This genre of epistles shows a side apart from the philosophy of the era. The examination of conscience begins with this letter writing. Diary writing comes later” (p. 30)
Foucault refers here to what he later describes as the hupomnema, the writing of the self into being.
Foucault, M. (1988). Technologes of the Self. In L. Martin, H. Gutman & P. Hutton (Eds.), Technologies of the self. Massachusetts: Massachusetts Press.