Gary - Please note that another poster mentioned the part about being prepared to teach at the highest level. I responded to that post with an observation, nothing more.
I wrote that the Latin root of the word "doctor" is "teacher", and that traditionally a doctorate is the degree required to join a university faculty. I guess I was a bit fast and loose with phrasing -- I didn't mean to imply that a doctorate is the
only necessary qualification to be a professor, just that it is generally the only degree required.
But as we pointed out, first-year MD graduates are teaching medical students on the wards of university hospitals. First-year PhD graduates are often running their own university-level classes and supervising PhD candidates in the lab.
But the real thrust of my post was that these things have developed over a millenium of tradition, and have very little logic and even less consistency behind them. There are universities where advanced graduate students can run their own intro courses, though in most they're relegated to being TA's. There are universities like the University of Chicago which don't have departments of engineering or journalism, because those fields are too practical for academic study -- instead, they recommend students take english or physics and offer simultaneous career development programs. (Though UC does have law and medical schools.) On the other hand, there are schools that have departments of hotel and restaurant management.
And there are whole fields that are exceptions to the "doctorate required to teach at University" rule. The classic example is the MFA -- up until recently the terminal degree in the fine arts. More programs are now awarding PhDs in fine arts, but that's a recent, politicalized change.
The only point I'm trying to make is that a degree only means what that particular degree from that particular university means. Good luck trying to explain to someone what the difference between an MSc (master of science), MSt (master of studies), and MPhil (master of philosophy) in a science field at Oxford means -- and yes, they do give out all three. They just mean what they mean, and just to understand one university's system takes looking at its particular history.
I feel like we're pulling up a lot of weird posturing to rationalize something that can't be rationalized. I meant the "a doctorate is what's required to teach at the highest level" in the most general sense -- it's the literal meaning, and usually it's the only academic qualification required to supervise a doctoral level graduate student.
This is like saying "a private pilot certificate is requirement to fully utilize the NAS." Yes, there are "lower" certificates that open up vast amounts of the NAS that for many people are "just as good". And yes, somebody with a PPL can't just go flying off to JFK without a few legal requirements (like a BFR and a medical) and a few "really good ideas" (like familiarity with complex airspace and ATC). And an IR is going to open up a whole new world. But the PPL is the basic qualification to be a pilot, just like a doctorate is the basic qualification to be an academic. And I'm happy to try to point out all of the weird idiosyncrasies of academia just like Ron Levy is happy to point out all of the weird idiosyncrasies of the FARs.