I occasionally get some cruel and bitter criticism from an odd source. I’m putting my response here for two reasons: (1) so that I can simply refer them to it and not have to repeat myself or engage in the equally impersonal displeasure of internet arguments, and (2) I think there is something interesting to be learned about mathematics, logic, and knowledge more generally.
It all started when I wrote a very controversial book about an extremely taboo topic: mathematics. In my book ABCs of Mathematics, “P is for Prime.” The short, child-friendly description I gave for this was:
A prime number is only divisible by 1 and itself.
I thought I did a pretty good job of reducing the concept and syllables down to a level palatable by a young reader. Oh, boy, was I wrong. Enter: the angriest group of people I have met on the internet.
You see, by the given definition, I had to include 1 as a prime number since, as we should all agree, it is divisible only by 1 and itself. Big mistake. Because, apparently, it has been drilled into people’s heads that this is a grave error, a misconception that can eventually lead young impressionable minds to a life of crime and possibly even death! It might even end up on a list of banned books!
By a vast majority, people love the book. I am generally happy with the response. The baby books I write are not for everyone — I get that. And I do try to take advice from all the feedback I receive on my books. There is always room for improvement. But the intense emotions some people have with the idea of 1 being a prime number is truly perplexing. Here are some examples:
I actually love the book, but there is a big mistake. The number 1 is not a prime number! The book should not be sold like this and needs to be reprinted.
1 IS NOT PRIME! How could a supposed math book have an error like this in it? I am disgusted!
Yikes. So what gives? Is 1 prime, or not? The answer is: that’s not a valid question. Let me explain.
First, let’s look at a typical definition. Compare to, for example, Wikipedia’s entry on prime numbers:
A prime number (or a prime) is a natural number greater than 1 that cannot be formed by multiplying two smaller natural numbers.
Much more precise — no denying that. It’s grammatically correct but probably hard to parse. I wanted to avoid negative definitions as much as I could in my books. But that’s beside the point. The reason 1 is not a prime is that the definition of prime itself is contorted to exclude it!
OK, so why is that then? Well, the answer is probably not as satisfying as you might like: convenience. By excluding 1 as prime, one can state other theorems more concisely. Take the Fundamental Theorem of Arithmetic, for example:
Every integer greater than 1 either is a prime number itself or can be represented as the product of prime numbers and that, moreover, this representation is unique, up to (except for) the order of the factors.
Now, this statement would not be true if 1 were a prime since, for example, 6 = 2 × 3 but also 6 = 2 × 3 × 1 and also 6 = 2 × 3 × 1 × 1, etc. That is, if 1 were prime, the representation would not be unique, and the theorem would be false.
However, if we do choose to include 1 as a prime number, all is not lost. Then the Fundamental Theorem of Arithmetic would still be true if it were stated as:
Every integer is a prime number itself or can be represented as the product of prime numbers and that, moreover, this representation is unique, up to (except for) the order of the factors and the number of 1’s.
Which version do you prefer? In either case, both the definition and theorem treat 1 as a special number. I’d argue that in this context, the number 1 is more of an annoyance that gets in the way of the deeper concept behind the theorem. But in mathematics, you must be precise with your language. And so, 1 must be dealt with as an awkward special case no matter how you slice it.
So, is 1 prime, or not? Well, it depends on how you define it. But in the end, it doesn’t really matter, so long as you are consistent. And understanding that is a much bigger lesson than memorizing some facts you were told in grade school.
The definition given in ABCs of Mathematics is not “wrong” any more than all of the other simplifications and analogies I have made are “wrong.” But, in case you were wondering, the second printing has been modified with the hope that everyone can enjoy the book. Even the angry people on the internet deserve to be happy.