Thinking he knows can be a trap. An ex-professor once told him he had a diamond-hard intellect and he'd been flattered at the time. Now he considers the nature of diamonds. Although sharp and glittering and useful for cutting glass, they shine with reflected light only. They're no use at all in the dark
The speaker reflects on the idea that believing one knows everything can be misleading. An ex-professor once praised him for his intellectual abilities, but he has come to question this compliment. The professor's words initially flattered him, but upon deeper consideration, he sees this self-assuredness as potentially hazardous.
The comparison to diamonds illustrates his point: while they may be strong and beautiful, they only reflect light and serve no purpose in the dark. This metaphor suggests that knowledge without awareness of its limitations might be useless in difficult circumstances, highlighting the need for humility and a true grasp of one's understanding.
