doctor

The root doctor goes back to the Latin verb docēre, "to teach." From it Latin built two key words: doctor, literally "a teacher" (one who teaches), and doctrīna, "that which is taught" — teaching, instruction, learning.

In the medieval universities, a doctor was the highest kind of teacher: a master scholar licensed to teach others. This is the original sense, still alive in the academic title PhD — a Doctor of Philosophy is literally someone learned enough to teach. The doctorate is the degree that certifies this learning. Only later did doctor narrow, in everyday English, to mean a physician — but think about why: a physician was a "learned person," someone with the scholarly knowledge of healing. The word didn't change its core idea (learning); it just attached to one prestigious profession.

From doctrīna ("what is taught") came doctrine — the body of teachings a church, party, or school holds and passes on. Push that one step further and you get doctrinaire: someone who clings to doctrine so rigidly that theory overrides reality — a person ruled by what they were taught rather than by what works.

The adjective docile comes from a sibling form, docilis, "easily taught." A docile student absorbs teaching without resistance; over time the word slid from "teachable" to "obedient, submissive" — the picture of someone who quietly does as they are told.

Finally, Latin documentum meant "a lesson, an example, something that teaches or proves a point." A document was originally a teaching example — proof you could point to. English kept the "proof" half and dropped the classroom: a document is now a written record that demonstrates or evidences something, and documentation is the body of such records.

So the whole family circles one idea — teaching and the knowledge that comes from it: the teacher (doctor), the learning (doctrine), the teachable (docile), and the proof you can teach from (document).