This is my second attempt to explain this little jewel taught to me by Dr. Dan Garber, professor of Engineering at the University of Maryland (sometime around 1979). See my original attempt.
A couple of people have commented “doesn’t the yellow circle represent your current knowledge better than the blue circle?” Answer: “No.” 🙂 First, the total body of knowledge is roughly infinite and thus needs to have infinite dimensions, and is therefore represented by the infinite plane that the circle is drawn on. The area inside the circle represents your current knowledge. In this metaphor, it would need to be an area covering gained knowledge, not a line with zero area. As you grow older and gain more knowledge, the circle expands and the area grows. [Obviously, a more sophisticated model would be n-dimensional and much more uneven than a circle, but let’s not go there. ;-)] The yellow circle is the edge between what you know and what you don’t know. You can think of the edge as your `knowledge horizon.’ Beyond the yellow is the stuff that you don’t know. And, so the point is that as your knowledge grows (area in the circle), your `knowledge horizon’ also grows, and thus your perception of how much you still have to learn grows too.
Again, it’s simple, maybe a little trite, and cutesy, but it reminds me of Dr. Garber’s kind, fatherly mentoring and reminds me not to get carried away with my own brilliance. 😉