Time to catch up on our blackboard quotes.
Learning without thought is labor lost;
thought without learning is perilous.
I have no particular talent, I am only inquisitive.
I advise my students to listen carefully the moment they decide to take no more mathematics courses. They might be able to hear the sound of closing doors.
— James Caballero
A mathematician’s work is mostly a tangle of guesswork, analogy, wishful thinking, and frustration. And proof, far from being the core of discovery, is more often than not a way of making sure that our minds are not playing tricks.
And one of my favorite Grooks:
Problems worthy
of attack
prove their worth
by hitting back.
Denise Gaskins' Let's Play Math 
Can you tell me who in the world is James Caballero? I’m guessing that he is a living mathematician/academician?
That sounds like a good guess. I can’t remember where I got the quote. I have a huge notebook of quotes I have copied at various times, and unfortunately I was not always careful to jot down my source. (It was probably from the Furman University server, which says the quote came from the CAIP Quarterly, Fall, 1989.)
When I put this post together, I did a search with Mr. Caballero’s name, but I could not find a reference for him, except for many other copies of this quote.