The term man of letters, as I understand it, was used in the 18th and 19th centuries to describe an individual who lived a marked intellectual life; who might, for example, own a large library, conduct independent scientific investigations on his own initiative, and be engaged in correspondence with leading literary and scientific figures of his time.
However, I have been confused for a time to what the letters in the appellation refer to. I have 3 hypotheses, in order of my confidence in them:
A man of letters is referred to as such because of the voluminous correspondence such a man would be expected to keep, as befitting a man acquainted with the leading literary and scientific figures of his day; we might expect the study room of a man of a letters to be replete with mail from all over the country, and much of his intellectual oeuvre might be completed within such letters. There are many prominent examples of this sort of thing; Darwin's letters contain much original work, as did Alexander Hamilton's.
The appellation to refers to the distinct British habit of abbreviating all sorts of personal and professional accomplishments with letters of the Latin Alphabet. For example, an Alexander Trevelyan, BA, MA, Ph. D, FRS, KB might be appropriately described as a man of letters.
It refers to simply the sort the material that should fill such a man's life; books, pamphlets, tractates and so on. The letters refer more to his literacy, than to his correspondence.
Which, if any of these, is correct?