by James Wallace Harris, 8/2/26
Euclid is sometimes called the Father of Geometry because of his book The Elements. A literal translation of the word geometry could be “earth measurement.” Even Aristotle knew that the Egyptians and Babylonians used geometry to survey land and construct buildings. Euclid doesn’t use numbers or make measurements in The Elements, and his geometry does not match what is taught in Geometry classes today.
What Euclid teaches is an abstract construction, yet it drives a pylon into reality. What Euclid really teaches is how to think clearly and precisely, something humans seldom do. There is a story about Abraham Lincoln that claims he didn’t become a good lawyer until he mastered the first six books of The Elements. (If you understand the foundation of what Euclid teaches, you will follow that link. If you don’t follow the link, you will after you have studied Euclid.)
The reason I’m studying Euclid’s Elements in retirement is that it’s exercise for my aging brain. Crossword puzzles, Wordle, and Sudoku are decent stretching exercises for my flabby neurons, but The Elements is Nautilus machines. I’m only as far as Proposition 12 in Book 1, which has left my brain in a puddle on the floor.
It’s not that the problems in The Elements are hard; they mostly require persistence. Numbers and measurements aren’t involved. Just logic, reason, a straightedge, compass, and pencil. Even a tiny amount of exercise can make you feel weak and lightheaded if you’re out of shape.
At 74, the challenge is whether I can learn anything new at all. Especially since I can’t seem to retain new words and concepts. I can still recall old stuff, but keeping new stuff is difficult. Maybe studying Euclid will help with that.
Before I retired in 2013, I thought I’d work on a degree in computer science as a hobby once I retired. I’ve taken many computer courses at the undergraduate level while earning an English degree. Since I spent my working life with computers, I always wanted a computer degree to validate myself. However, when I investigated the requirements for a B.S. or M.S. in computer science, I learned that I needed 24 hours of math as a prerequisite.
I only had College Algebra, Calculus I, and Statistics from the 1970s. When I looked at textbooks in those courses, I realized I had forgotten everything. I’d need to start over from scratch. Not only that, but when I tried Khan Academy as a refresher, I learned I had forgotten the fundamentals of math from elementary school. I had to start over in the 2nd grade. I eventually worked my way up to 5th grade in Khan Academy, but my brain ran out of gas. I dropped the idea of going back to college as a retirement hobby.
Twelve years later, my mind is even more mushy. I do Wordle and the Mini Crossword at The New York Times every day. Even though I often have trouble remembering words, I’m pretty good at those puzzles. I gave up on learning math over a dozen years ago. I regret that. I should have stuck it out.
Recently, I read about the history of Euclid’s Elements. Supposedly, it’s only second to The Bible in the number of editions over the last two thousand years. Its main claim to fame is that it teaches more than basic geometry – it pushes its students to think logically.
The geometry in Elements is very basic, not like the geometry they teach in school today. All 13 books of the Elements use the same 5 Postulates and 5 Common Notions. From that foundation, 48 propositions (problems) are set up in Book 1. Each proposition is proved step by step with diagrams based on the 23 definitions. 131 total definitions cover 465 propositions in all 13 books. Most students only do the first 6 books.

For centuries, an educated person in the Western world would study Latin, Greek, and Euclid’s Elements. Before the scientific method, most knowledge was opinion. Euclid’s Elements set up the idea that knowledge should be built on a foundation. A foundation that is consistent across cultures. Every proposition has to be proved with definitions, postulates, and common notions. Look at this video; the visual demonstration will probably say more than I can in words.
The simplicity and limited domain of the Elements appealed to me. I may not be able to pass a university math course, but hopefully, I can work my way through the first book of the Elements.
I started with the free web version, and then bought a 99-cent Kindle edition of The Elements of Euclid for the Use of Schools and Colleges (Illustrated) so I could read on my phone. I also bought Delphi Collected Works of Euclid (Illustrated) because it had a long history of this famous book. It was only $2.99 for the Kindle edition, and I liked the scholarly history of how Euclid wrote the Elements and how people used it over the centuries.
I’ve been able to work through the early Propositions. I even bought a compass to draw them out myself. I’m not sure I’d even call this mathematics. It’s really just logic. The work is tedious, though. With each step, you’re supposed to cite the Definition, Postulate, or Common Notion you used to justify that step. You have to think hard as you follow along.
Here’s my work for Proposition 1. Most books describe the steps in words, and then show the final diagram. I find that confusing.
Proposition 1: To construct an equilateral triangle on a given finite straight line.
Look at how David E. Joyce works the problem at Clark University. It’s very wordy, but it also explains some of Euclid’s faults. But even Euclid’s description is very confusing, at least to me. It’s only clear when you follow the problem logically, step by step.
We start with a line segment with the endpoints being A and B. Ignore point C for now. Many teachers start by drawing line segment AB and immediately put a point C above it. I think that’s wrong.
Draw:

Then draw a circle with A at the center, with AB as the radius.

Then create a circle with B as the center, using AB as the radius.

Now, it’s time for point C. Where the circles intersect, designate points. Remember the definitions. They are abstractions, but we have to always build on them.
Definition 1: A point is that which has no part.
Definition 2: A line is a breadthless length.
Definition 3: The ends of a line are points.
Definition 15: A circle is a plane figure contained by one line such that all the straight lines falling upon it from one point among those lying within the figure equal one another.
Definition 16: And the point is called the center of the circle.
Postulate 3: To describe a circle with any center and radius.
Euclid doesn’t state that when a line crosses another line, the intersection is a point, but we assume it. He should have included it as a definition.
Because the two circles intersect, we have defined two additional points, which we can label C and D; however, D is not needed for this proof.

Now we can draw lines AC and BC.

Definition 20: Of trilateral figures, an equilateral triangle is that which has its three sides equal, an isosceles triangle that which has two of its sides alone equal, and a scalene triangle that which has its three sides unequal.
Postulate 1: To draw a straight line from any point to any point.
Common Notion 1: Things which equal the same thing also equal one another.
Common Notion 4: Things which coincide with one another equal one another.
Because AB is the radius of both circles, and AC and BC are also radii of those same circles, all sides of the triangle are equal.
Thus we have proved:
Proposition 1: To construct an equilateral triangle on a given finite straight line.
In modern geometry, we would have just drawn an equilateral triangle knowing that all three angles equal 60 degrees. And we could have measured the length of each side with a ruler. The point of Euclid is to create geometric objects with basic geometric building blocks. No numbers are involved.
The rest of The Elements involve creating ever-evolving complicated 2D and 3D geometric shapes. Some of which prove other mathematical problems, like Proposition 47. It’s an alternate method of proving the Pythagorean theorem.
I’ve been able to work through the early Propositions. I even bought a compass to draw them out myself. I’m not sure I’d even call this mathematics. It’s really just logic. The work is tedious, though. With each step, you’re supposed to cite the Definition, Postulate, or Common Notion you used to justify that step. It’s much easier to watch a Proposition being proved with a YouTube video.
But even after watching a video multiple times, it’s still hard to do it myself. Without practicing with drawing lines and circles, it’s hard to see how the proofs really work, even when they are shown to you.
JWH




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