Dobbs: a telling problem

Another memory from Bob Dobbs:

After the end of the formal part of a Monday Night “performance” in 1978, I was eavesdropping on Marshall McLuhan engaging in some intellectual banter with a Professor. 
They were getting into the dialogue deeper and deeper when McLuhan all of a sudden said to his “colleague”:
“You’re trying to TELL me something.”

This was the end of their exchange.