In Gauss's day, the number 1 was indeed considered a prime number. It couldn't be split into a product of two smaller numbers. But as mathematicians realised that the primary importance of prime numbers was as the building blocks for all numbers, the status of 1 as a prime declined. The number 1 doesn't have the same quality as the other prime numbers. It doesn’t help build numbers. If I multiply a number by a prime then I build a new number. In contrast if I multiply by 1, I get nothing new. This brick seems to be made of air. So there are good reasons to exclude 1 from our list of primes.