After some discussion as to what “essential object” meant, the professor leading the seminar said something meant to clarify things and drew something that looked like lightning bolts on the blackboard. “Mr Feynman,” he said. “Would you say that an electron is an essential object?”

Well, now I was in trouble. I admitted that I hadn’t read the book, so I had no idea of what Whitehead meant by the phrase; I had only come to watch. “But,” I said, “I’ll try to answer the professor’s question if you will first answer a question from me, so I can have a better idea of what ‘essential object’ means. Is a brick an essential object?”

[…] Then the answers came out. One man stood up and said. “A brick as an individual, specific brick. That is what Whitehead means by an essential object.”

Another man said, “No, it isn’t the individual brick that is an essential object; it’s the general character that all bricks have in common – their ‘brickiness’ – that is the essential object.”

Another guy got up and said, “No it’s not in the bricks themselves. ‘Essential object’ means the idea in the mind that you get when you think of bricks.”

Another guy got up, and another, and I tell you I have never heard such ingenious different ways of looking at a brick before. And, just like it should in all stories about philosophers, it ended up in complete chaos. In all their previous discussions they hadn’t even asked themselves whether such a simple object as a brick, much less an electron, is an essential object.

Richard Feynman, “Surely You’re Joking, Mr Feynman!”