When a word is capitalised in the middle of a sentence (not the beginning, not in the title, etc), is it necessarily a proper noun?
Do we have grammatical rules for capitalising a word, which is not a proper noun, in the middle fo a sentence?
Not all capitalized words are proper nouns. Words in the middle of a sentence can be capitalized for various reasons. Jason Bassford pointed out that the first-person singular pronoun I is always capitalized, even though it is generally not classified as a proper noun. Some adjectives related to proper nouns are capitalized, such as French and Christian in the following sentence:
Points of narrative return and departure, they comprise fantasies of itineraries that the very French and very Christian subjects of the novel will make in the world at large.
("There's a New World Here: Pantagruel via Oronce Finé", by Tom Conley, in French Global: A New Approach to Literary History, edited by Christie McDonald and Susan Suleiman, p. 35)
The use of very makes it clear that these words are adjectives and not nouns in this context. (In other contexts, Christian can be a noun, as in the sentence "You are a Christian.")