Walter Ong, Orality & Literacy:

The personal diary is a very late literary form, in effect unknown until the seventeenth century… The kind of verbalized solipsistic reveries it implies are a product of consciousness as shaped by print culture. And for which self am I writing? Myself today? As I think I will be ten years from now? As I hope I will be? For myself as I imagine myself or hope others may imagine me? Questions such as this can and do fill diary writers with anxieties and often enough lead to discontinuation of diaries. The diarist can no longer live with his or her fiction.