Doctor in Latin means "teacher", with (I think) connotations of being learnèd or highly educated, as in Philosophiæ Doctor. How did it acquire its modern English meaning of a licensed physician?
Owen Barfield, in History in English Words, p. 100, suggests that the character of Ancient Roman society, with its emphasis on physicality rather than introspection, somehow influenced the word "doctor" to become specialized to "a teacher of physical health" in modern English.
Etymonline, apparently summarizing the OED, reports that Latin doctor first came to mean one who had received the highest degree from a university, hence qualified to teach; then any professional licensed by passing exams; and then became specialized to mean one licensed to practice medicine.
Can anyone provide more detail about how doctor came to its present meaning? In particular, is there any truth in Barfield's suggestion that doctor ever, in Latin or English, was understood as "a teacher of physical health"?