Tyler Cowen wrote in his book Discover Your Inner Economist, :

If a book lists ‘Ph.D.’ after the author’s name, be wary. The author needs those letters to signal his importance. It usually means the author is not used to interacting with peers, is appealing to the gullible, or is making questionable claims. Stephen Hawking does not use those three letters.

Cowen was talking about books, but his point about people needing their letters for credibility is relevant to martial arts.

My adult students call me Alex. My kid students call me Alex or Coach Alex. Nobody calls me Professor.

My teacher is named Alex. I call him Alex. When I see Ryron or Rener Gracie, I call them Ryron or Rener.

I train with friends who are doctors. I don’t call them Dr. John or Dr. Jane. And then I also train with people who are kind of like doctors, and they insist on being called Dr. Whomever.

When I was learning Wing Chun, I had to call my teacher Sifu. When I had a Tae Kwon Do teacher subleasing my school for his classes, I saw that his students and their parents called him Master. He insisted I call him Master too, but that seemed weird so I didn’t.

I think the terms Professor, Master, Sifu, etc. get lost in translation. But I really like the casualness of jiu-jitsu. I respect my teachers and they earn it with their mastery. The real ones don’t need letters and titles.