I can attest that there is a such thing as a doctor. I saw one just a couple of weeks ago. I'm pretty sure she specializes in family practice, but she is a doctor none the less. And she'd probably be be very confused if I told her she didn't exist, or she wasn't a doctor because doctors don't exist.
But she wouldn't be confused if you said she wasn't a heart surgeon, because she's not. She can tell you a bunch of stuff about the heart, she can probably do surgery if she was in a position where she had to, but she's not a heart surgeon and wouldn't claim to be.
