Who are the "Reality Instructors" and what is their thematic significance in Saul Bellow's Humboldt's Gift?
Saul Bellow coined the term "reality instructor" to mean any person who takes their own superiority for granted and then spends their time trying to "educate" others in their philosophies. Bellow intended the term to be a pejorative, because that sort of person is usually too blinded by their own prejudice to properly recognize objective reality, instead interpreting everything to their own ideas.
In his novel Humboldt's Gift (1975), Bellow has Charlie (loosely based on himself) weather any number of setbacks, including a low-level gangster -- Cantabile -- who wants to mooch off Charlie's success. Cantabile is also jealous of Charlie's intellect, although he wants only to profit off it, not possess it himself:
"I saw the look you gave when I told the name of the cow-college I went to."
"What's colleges got to do with it?"
"Don't you understand... you've written all that stuff. You're in Who's Who. But you dumb asshole you don't understand anything."
I must have received ten such calls from Rinaldo Cantabile.
(Bellow, Humboldt's Gift, Google Books)
Cantabile serves as a "reality instructor" for Charlie, as he appears when Charlie is getting puffed up or smug about his own success, and although Cantabile means only to use Charlie, he proves useful near the end of the book when he helps Charlie get money from a plagiaristic film company. Thus, his "reality" was able to serve a purpose, although Charlie rejects his negative influence thereafter.