Short answer: 1 could be counted as a prime number, and in fact it used to be counted as a prime number by some mathematicians [1]. These days, most people just

Short answer: 1 could be counted as a prime number, and in fact it used to be counted as a prime number by some mathematicians [1]. These days, most people just choose not to do that, since this makes most proofs and definitions more convenient. Shorter answer: Convenience. [1] http://mathworld.wolfram.com/PrimeNumber.html http://mathworld.wolfram.com/PrimeNumber.html (3rd paragraph)