Hawthorne is never shy about self-interpreting. If there is a biblical allusion, symbol, or allegorical meaning to explain, he will usually explain it for you. See for example “The Minister’s Black Veil,” in which the significance of the minister’s mysterious black veil gets more or less spelled out at the end:

Why do you tremble at me alone?“ cried he, turning his veiled face round the circle of pale spectators. "Tremble also at each other. Have men avoided me and women shown no pity and children screamed and fled only for my black veil? What but the mystery which it obscurely typifies has made this piece of crape so awful? When the friend shows his inmost heart to his friend, the lover to his best-beloved; when man does not vainly shrink from the eye of his Creator, loathsomely treasuring up the secret of his sin,—then deem me a monster for the symbol beneath which I have lived and die. I look around me, and, lo! on every visage a black veil!

Hawthorne’s framing device of the fictitious author is quite elaborate. “M. de l'Aubepine” is given a partial bibliography as well as a lengthy introduction. This makes the author seem more “real,” and also makes the self-parody and self-critique (as explained above) more poignant.

Hawthorne was likely inspired to try this sort of framing device by reading his predecessors among the English Romantic writers, who tended to frame their “ghost stories.” Look at Daniel Defoe’s “A True Relation of the Apparition of One Mrs. Veal the Next Day after Her Death to One Mrs. Bargrave at Canterbury the 8th of September 1705,” and at the Preface of Horace Walpole’s The Castle of Otranto (1764).

Hawthorne’s technique here may have influenced the 20th-century Argentine writer Jorge Luis Borges, who took such metafictional games to even greater extremes. In a famous essay on Hawthorne, Borges ponders the paradoxes of literary influence:

…a great writer creates his precursors. He creates and somehow justifies them.

Reference to Nicolas Malebranche (1638 – 1715), French rationalist philosopher.

Malebranche argues in the Dialogues on Metaphysics, a dialogue between Theodore and Aristes, that we do not have a complete conception of the powers of the mind, and thus no clear conception of the nature of the mind.

I am unable, when I turn to myself, to recognize any of my faculties or my capacities. The inner sensation which I have of myself informs me that I am, that I think, that I will, that I have sensory awareness, that I suffer, and so on; but it provides me with no knowledge whatever of what I am – of the nature of my thought, my sensations, my passions, or my pain – or the mutual relations that obtain between all these things […] I have no idea whatever of my soul.

This leads Theodore to declare that ‘I am not my own light to myself’; the nature of our own minds is highly obscure. (Source)

O'Connor here draws a delicate and perhaps telling parallel with her own life. Like her father, she had systemic lupus erythematosus (lupus), an incurable condition that often proves fatal. Diagnosed in 1951 while still in her mid-twenties, she was predicted to live five more years. In fact she lived fourteen, but died before reaching age 40. The illness kept her housebound for much of her adult life.

“Dinner” here is used in the old-fashioned sense of “lunch.” Nowadays, of course, it’s synonymous with “supper.”