A student in class today asked why we capitalize the first person singular subjective-case personal pronoun, I, but none of the other personal pronouns. “Why don’t we capitalize ‘he’ or ‘she’?” the curious young lady asked.

Indeed.

“I’ll look into that,” I replied, scribbling in my little notebook.

The New York Times offers an answer:

England is where the capital “I” first reared its dotless head. In Old and Middle English, when “I” was still “ic,” “ich” or some variation thereof – before phonetic changes in the spoken language led to a stripped-down written form – the first-person pronoun was not majuscule in most cases. The generally accepted linguistic explanation for the capital “I” is that it could not stand alone, uncapitalized, as a single letter, which allows for the possibility that early manuscripts and typography played a major role in shaping the national character of English-speaking countries. (New York Times)

The whole article is quite interesting.