Edgar asked exactly that question--well, not exactly but pretty close:
(Found this on the www.)

"Why Aren't There Negative Prime Numbers?


Date: 12/10/1999 at 00:12:32
From: Edgar Lopez
Subject: Prime Numbers

Can negative numbers be prime numbers?

In my math book it says that a prime number is a whole number greater
than 1 that has only two factors, 1 and itself. So why can't, for
example, (-5) be prime? Its factors would be -5 and 1. I don't get it.

Is it because 5 and (-1) are also factors of (-5)? I say no because
(-1) and (-5) would then be factors of 5, and 5 wouldn't be prime. I
don't understand and neither does my teacher. We tried to figure it
out, and then she told me to ask you.

Date: 12/10/1999 at 19:24:14
From: Doctor Ian
Subject: Re: Prime Numbers

Hi Edgar,

This is an excellent question, but in order to understand the answer,
you have to get a feel for what mathematics is about. It's not about
cranking numbers through equations and getting answers, and it's not
about balancing your checkbook or figuring out how long it will take
Bill and Janet to mow the lawn if they work together, and it's not
about building bridges or telephones or sending spaceships to other
planets. Those are all _uses_ of math, but mathematics itself is about
searching for patterns.

The most interesting patterns are the ones that hold for the largest
classes of numbers. So something that is true for all numbers is more
interesting than something that is true for just integers, or just
prime numbers, or just numbers smaller than 10, or just the number 17.

(Note that 'patterns' are sometimes called 'theorems,' and sometimes
called 'properties'. For example, the prime number theorem, which says
that any composite number can be broken into a product of primes, is
one kind of pattern. The commutative property of addition, which says
that you can add numbers in any order, is another kind of pattern.)

Before anyone figured out the need for negative numbers,
mathematicians had already discovered lots of patterns involving prime
numbers. So when negative numbers came along, making some of them
prime would have caused a lot of patterns (patterns that looked like
"For any prime number, blah blah blah") to stop being true. That would
have been annoying without serving any real purpose, and so the
definition of primes was adjusted to exclude negative numbers.

This, by the way, is why zero and one aren't prime numbers either,
although you can make some good arguments for why they should be. If
they were considered prime, then a lot of patterns that now look like
"For any prime number, blah blah blah" would have to be changed to
"For any prime number except one or zero, blah blah blah." That makes
the patterns uglier, and harder to remember. ("Oh, wait - does this
pattern for prime numbers also apply to zero?")

Mathematics turns out to have so many practical uses that it's easy to
forget that the mathematicians who are creating it, in many cases,
really don't care whether the patterns they are finding have any more
uses than a song, or a painting.

I hope this helps. Thanks for asking an interesting question. Be sure
to write back if you have others.

- Doctor Ian, The Math Forum
http://mathforum.org/dr.math/ "